2014

U.S. Department of the InteriorU.S. Geological Survey

Prepared for the National Aeronautics and Space Administration

Geologic Map of MarsBy Kenneth L. Tanaka, James A. Skinner, Jr., James M. Dohm, Rossman P. Irwin, III, Eric J. Kolb, Corey M. Fortezzo, Thomas Platz, Gregory G. Michael, and Trent M. Hare

Pamphlet to accompany

Scientific Investigations Map 3292

View of the northern part of the western hemisphere of Mars. Left half shows a color elevation, shaded relief view highlighting the immense shields of the Tharsis rise. Right half shows a true-color view of the vast Valles Marineris and Kasei Valles canyon systems, which connect to the dark basin of Chryse Planitia at upper right. (Image data from NASA.)

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ContentsIntroduction ....................................................................................................................................................1Physiographic Setting ...................................................................................................................................2Data ..... ............................................................................................................................................................3Mapping Methods..........................................................................................................................................3Map Development and Author Roles .........................................................................................................4

Unit Delineation .....................................................................................................................................4Unit Groups ............................................................................................................................................5Unit Names and Labels ........................................................................................................................5Contact Types ........................................................................................................................................5Line Feature Types ................................................................................................................................6Drafting ..................................................................................................................................................6

Age Determinations .......................................................................................................................................6Geologic History .............................................................................................................................................9

Noachian Period ...................................................................................................................................9Early Noachian Epoch ........................................................................................................................10Middle Noachian Epoch ...................................................................................................................10Late Noachian Epoch ........................................................................................................................11Hesperian Period ................................................................................................................................12

Early Hesperian Epoch ..............................................................................................................12Late Hesperian Epoch ...............................................................................................................13

Amazonian Period ...............................................................................................................................13Early Amazonian Epoch ...........................................................................................................14Middle Amazonian Epoch ........................................................................................................14Late Amazonian Epoch .............................................................................................................15

Major Results................................................................................................................................................15Acknowledgments .......................................................................................................................................15References Cited..........................................................................................................................................16

Tables1. Martian epoch lower boundary ages..................................................................................................25 2. Assigned model ages for selected Mars global map units ..........................................................263. Total crater densities for map units on Mars ................................................................................324. Impact craters and basins (and possible impact-like features of other origin) on Mars .........345. Basis for time-stratigraphic assignments for each map unit as shown in the Correlation of Map Units. ................................................................................................................ 376. Percentages of previously mapped Viking-based map units for each unit in this Mars global map ..................................................................................................................397. Percentages of map units in this Mars global map for each previously mapped Viking-based map unit .................................................................................................41

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IntroductionMars is the fourth planet from the Sun and the outermost of

the rocky, terrestrial planets that make up the inner solar system. Of this planetary family, Mars is only larger than Mercury. Surface gravity on Mars is 3.71 m/s2, which is 37.6 percent that of the Earth’s. The present atmospheric pressure is low (~0.6 kPa) relative to Earth’s (101 kPa), and the atmosphere is mostly carbon dioxide (95%). The obliquity of Mars is presently 25° and may have varied by tens of degrees over the past tens of millions of years and longer (Laskar and others, 2004).

The Martian polar ice caps, observable from Earth, are made up mostly of water ice. Thus the possibilities of liquid water and habitable environments on or near the surface of Mars have driven interest in the planet (for example, Des Marais, 2010). Orbiting at a distance on average about 50 percent far-ther from the Sun than the Earth, Mars is the only other planet besides the Earth that is within the solar system’s potential habitable zone (Kopparapu and others, 2013).

Beginning in the late 1960s with the Mariner 4, 6, and 7 fly-bys, spacecraft began imaging the planet from closer perspectives (Snyder and Moroz, 1992). Mariner 9 (1971–72) was the planet’s first orbiter, and it successfully imaged the entire globe at 2 to 3 km/pixel resolution. These early spacecraft revealed the planet’s Moon-like cratered highlands, immense volcanoes, huge tectonic rifts, and, strikingly, evidence for flowing water in the planet’s ancient past, based on identifica-tion of branching valley network systems and broad outflow channels. The next major phase of spacecraft exploration in the late 1970s and early 1980s involved the two Viking Orbiters and two Viking Landers. The orbiters imaged the surface at tenfold to hundredfold increased detail, and the landers performed the first biological experiments in search of Martian life (*, Klein and others, 19921). The most recent phase of Mars exploration, since the late 1990s, has led to significant increase in the types, spatial resolution, and amount of data returned both from Mars orbit and the surface (*, Barlow, 2008; Bell, 2008). Post-Viking orbiters include Mars Global Surveyor, Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter. These have deliv-ered high-resolution visible, thermal, and multispectral imaging; laser altimetry; radar, gravity, and magnetic sounding; and other measurements of the Martian surface, atmosphere, and crust. Rovers and stationary landers, including Mars Pathfinder, Spirit and Opportunity Mars Exploration Rovers, Mars Phoenix, and the Curiosity Mars Science Laboratory, have performed increas-ingly sophisticated surface investigations.

Each major phase of exploration has resulted in a global geologic map, which encapsulates the most significant advances in understanding of Martian surface evolution. The first global map of Mars was produced by Scott and Carr (1978) at a scale of 1:25,000,000 using Mariner 9 visible wavelength images (2–3 km/pixel resolution). That map included compilation of a partly completed, 30-quadrangle, 1:5,000,000-scale geologic map series drafted by various authors. Scott and Carr (1978)

identified 24 global geologic units among 5 terrain groups and established the 3 principal time-stratigraphic divisions for Mars—the Noachian, Hesperian, and Amazonian Periods (from oldest to youngest). The second global map of Mars was published in three parts at a scale of 1:15,000,000. Western and eastern equatorial regions (lat <57°) were produced by Scott and Tanaka (1986) and Greeley and Guest (1987), respectively. Polar regions (lat >55°) were produced by Tanaka and Scott (1987). Viking Orbiter images mostly between 100 and 300 m/pixel resolution served as the bases for these maps, which identified 90 map units, including units of local to global extent. Some of these units were used as referents to subdivide the time-stratigraphic periods of Scott and Carr (1978) into epochs (Early, Middle, and Late Noachian; Early and Late Hesperian; and Early, Middle, and Late Amazonian), as well as to establish the crater-density values for their boundaries (Tanaka, 1986). Both the Mariner 9- and Viking-based geologic maps were orig-inally drafted over shaded-relief-map bases that were manually airbrushed using reference images (Batson and others, 1979).

Herein, we present a new global geologic map of Mars, which records the distribution of geologic units and landforms on the planet’s surface through time, based on unprecedented variety, quality, and quantity of remotely sensed data acquired since the Viking Orbiters. These data have provided morpho-logic, topographic, spectral, thermophysical, radar sounding, and other observations for integration, analysis, and interpreta-tion in support of geologic mapping. In particular, the precise topographic mapping now available has enabled consistent morphologic portrayal of the surface for global mapping; whereas previously used visual-range image bases were less effective, because they combined morphologic and albedo infor-mation and, locally, atmospheric haze. Also, thermal infrared image bases used for the new map tended to be less affected by atmospheric haze, making them reliable for analysis of surface morphology and texture at even higher resolution than the topographic products. Moreover, following publication of the Viking-based global map of Mars, a multitude of geologic maps of different parts of the Martian surface at various scales using the Viking Orbiter and subsequent datasets were produced to address important questions regarding Martian surface geol-ogy and evolution; major results from these efforts, mostly at 1:1,000,000 and smaller (zoomed-out) scales, have been incor-porated herein (the locations of these maps are shown in fig. 2). Finally, a host of other published studies have contributed to the interpretations and geologic reconstruction contained herein. To keep our reference list at a reasonable length, we precede some citation lists with an asterisk (*, see footnote 1) to indicate that other useful, particularly earlier, publications exist.

We used digital photogeologic mapping techniques to identify and discriminate 44 geologic units (encompass-ing ~1,300 separate occurrences) and 12 linear feature types (>3,500 individual features mapped, not all shown on map, included in digital files), which collectively document major episodes of unit emplacement and modification. Along with cross-cutting relations, analysis of global as well as specific detailed impact crater size-frequency distributions provide an unprecedented documentation of relative and modeled absolute

1An asterisk (*) is included where only a small sample, generally recent, of the many representative publications are cited.

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ages of map units and their individual occurrences (see tables 1–3 and D12 at http://pubs.usgs.gov/sim/3292/ and fig. 1), as well as chronostratigraphic ages for all impact craters and basins >150 km in diameter (table 4). Key stratigraphic rela-tions and crater-density data are used to constrain the time-stratigraphic age assignments for each map unit as shown in the Correlation of Map Units; this information is summarized in table 5. Due to our focus on global (vs. regional) reconstruc-tions, we avoid dividing units based upon modest regional variations in age and physical character. These efforts result in an improved characterization of Mars compared to the previous, Viking-based mapping (tables 6 and 7) and, therefore, provide a refined portrayal of Mars’ geologic character and evolution.

Physiographic SettingThe global topography of Mars defines broad regions of

geologic significance that include geographic names established by the International Astronomical Union (IAU). We include only names of major features and a representative selection of others mentioned herein; see the topographic map of Mars (U.S. Geological Survey, 2003) and the Gazetteer of Planetary Nomenclature (http://planetarynames.wr.usgs.gov/) for more detailed information about and portrayals of Martian geographic features.

Densely cratered, rugged highlands cover much of the southern latitudes and some parts of the northern hemisphere of Mars, including Arabia Terra, Terra Sabaea, and Tempe Terra. Most highlands are between 0 to 5,000 m above datum, which is defined at Mars’ mean planetary radius (Smith and others, 1999, 2001). However, Arabia Terra and parts of other highland areas north of the equator lie at 0 to –3,000 m elevations. The northern plains lie even lower in elevation, mostly in the range of –4,000 to –5,000 m. They are much less cratered than the highlands, indicating that burial by younger rock and sediment sequences has obscured much of the earlier geologic record. The three largest, well-preserved impact basins in the Martian cratered highlands are Hellas basin (~2,400 km in diameter and includes the deepest surface point on the planet at –8,200 m elevation), Isidis basin (~1,500 km in diameter) along the high-land-lowland margin, and Argyre basin (~900 km in diameter). Utopia Planitia forms the floor of a giant (nearly 3,000 km in diameter) circular basin northeast of Isidis Planitia, interpreted

by many to be an ancient impact basin (*, McGill, 1989). Other circular but generally less well defined broad depressions that also may be impact structures occur across Mars (*, Schultz and others, 1982; Frey, 2006).

The zone of transition between the Martian highlands to the south and the lowlands to the north (mostly –1,000 to –4,000 m) is marked by diverse landforms, including scarps, knobs, mesas, and plateaus that are hundreds of meters to a few kilometers in relief. These landforms extend from the southern margins of Acidalia Planitia eastward to the southern margin of Amazonis Planitia. A gigantic system of linear canyons, Valles Marineris, forms along the northern margin of the Thaumasia plateau and extends for >2,000 km from Noctis Labyrinthus along the eastern margin of the Tharsis rise. The canyon system connects to the east with chaotic fields of knobs and mesas that mark the source regions of the broad outflow channels of circum-Chryse; the channels dissect highland surfaces to depths of several kilometers over areass that extend a few thousand kilometers prior to their entrance into Chryse Planitia (*, Rotto and Tanaka, 1995), where the Viking 1 Lander and Mars Pathfinder sites occur (*, Golombek and others, 1997; Crumpler and others, 2001).

Regionally focused, long-lived volcanism resulted in the formation of high-standing and broad rises and shields, pla-teaus, and local edifices. The Tharsis rise (or region) consists of an immense assemblage of flow fields that extend 5,000 km east-west by 6,000 km north-south. The rise is capped by some of the largest volcanic constructs in the solar system, includ-ing Olympus Mons, Alba Mons, and the three aligned Tharsis Montes (Ascraeus Mons, Pavonis Mons, and Arsia Mons). In addition, the Tharsis rise includes fields of smaller volcanoes, as well as tectonically deformed, elevated terrains that include parts of Tempe Terra, Noctis Fossae, Claritas Fossae, Ceraunius Fossae, and the Thaumasia highlands (*, Tanaka and others, 1991; Dohm and others, 2001a). The summit of Olympus Mons stands at 21,229 m—the highest elevation on Mars—and the mountain is surrounded by several blocky, lobate aureoles that extend for hundreds of kilometers. Other regional volcanic cen-ters include the Elysium rise (composed of Elysium Mons and surrounding volcanoes and lava-flow fields), Hesperia Planum and Malea Planum (northeast and southwest margins of Hellas Planitia, respectively), and Syrtis Major Planum (west margin of Isidis Planitia). Smaller volcanoes and volcano-like edifices ranging from tens to a few hundred kilometers in diameter are distributed across the southern highlands, with a particular con-centration south of the Tharsis rise.

Both of the Martian poles are capped by plateaus that are elevated a few kilometers above the surrounding plains. The circular, north polar plateau—Planum Boreum—occurs near the geographic center and low point (approx. –5,000 m eleva-tion) of the northern lowlands. Planum Boreum is ~1,000 km in diameter and is partly surrounded by vast dune fields, including Olympia Undae. The south polar plateau—Planum Australe—occurs within southern cratered highlands standing at 1,000 to 1,500 m elevation. Planum Australe has an ellipsoidal planimet-ric shape (~1,100 x 1,400 km). Both poles contain character-istic spiral troughs that expose the depths of the polar plateau,

2 Table D1 is a Microsoft Excel spreadsheet file in .xlsx format available online only. This table provides a more complete presentation of the crater size-frequency data and results for the same 48 counting areas provided in table 2. The additional information includes total numbers of craters; whether a resurfacing correction was used; diameter ranges used for model crater size-frequency fits; numbers of craters in fit ranges; cumulative densities and density errors of craters >1 km in diameter per million km2 for model-fit isochrons; actual cumulative crater densities for diameter steps at 0.5, 1, 2, 5, and 16 km; ratios of actual to expected values of cumulative densities, based on estimated densities projected from values at next larger diameter step; instrument that obtained the image data used; image numbers; and image resolutions.

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including one large reentrant in the north (Chasma Boreale) and three reentrants in the south (Chasma Australe, Promethei Chasma, and Ultimum Chasma).

DataThe primary base maps for this geologic mapping investi-

gation are the Mars Global Surveyor (MGS) Mars Orbiter Laser Altimeter (MOLA) digital elevation model (DEM) (463 m/pixel resolution at lower latitudes to 115 m/pixel near the poles) (Smith and others, 2001) and the Mars Odyssey (ODY) Thermal Emission Imaging System (THEMIS) daytime infrared (IR) image mosaic (100 m/pixel) (Christensen and others, 2004). Both data sets are registered to the spheroid of Mars, with an equatorial radius of 3,396.19 km and a polar radius of 3,376.2 km (Seidelmann and others, 2002). Longitude increases to the east, and latitude is planetocentric, in compliance with IAU (International Astronomical Union), International Association of Geodesy, NASA (National Aeronautics and Space Administration), and U.S. Geological Survey standards.

The MOLA-derived shaded-relief image used for the base represents more than 600 million single altimeter measurements gathered between 1999 and 2001, adjusted for consistency (Smith and others, 2001; Neumann and others, 2001, 2003), and converted to planetary radii values. Using these values, elevations above the areoid were determined from the Martian gravity field solution GMM-2B (Lemoine and others, 2001). The average accuracy of each point is originally ~100 m in hori-zontal position and ~1 m in radius (Neumann and others, 2001). However, the total elevation uncertainty is at least ±3 m due to the global error in the areoid (±1.8 m according to Lemoine and others, 2001) and regional uncertainties in its shape. The mea-surements were converted into a digital elevation model (DEM; Neumann and others, 2001) using Generic Mapping Tools soft-ware, with a resolution of 128 pixels/degree (or 463 m/pixel at the equator). The MOLA DEM does not contain data at lat >88° N. and S. due to the 92.8° inclination of the MGS orbit (Smith and others, 2001).

A shaded-relief image was generated from the DEM with a sun angle of 45° from horizontal and a sun azimuth of 335°, as measured clockwise from north.

The artificially illuminated MOLA DEM serves as the base for the printed geologic map. To provide a true global view on a single map sheet, Robinson projection was selected at 1:20,000,000 scale. Polar regions (lat >70° N. or S.) are represented in stereographic projection. The printed map scale forms the basis for determining the digital mapping parameters and the minimum size of mappable features, as discussed in the Mapping Methods section.

The THEMIS daytime and nighttime IR 100 m/pixel global mosaics (version 11; P.R. Christensen, N.S. Gorelick, G.L. Mehall, and K.C. Murray, THEMIS Public Data Releases, Planetary Data System node, Arizona State University, http://themis-data.asu.edu) represent thermal infrared energy emit-ted in nine wavelength bands, which relate to the tempera-ture and emissivity of the surface (Christensen and others,

2004; Edwards and others, 2011). For the daytime mosaic, about 21,000 THEMIS daytime images were selected with the following parameters: shutter closing time <150 seconds, incidence angle <85°, minimum surface temperature >160 K, and 0 percent saturated pixels from lat 90° S. to 90° N. with unsummed (full-resolution) data. For the nighttime mosaic, images were selected with the following parameters: shutter closing time <150 seconds, incidence angle >95°, minimum surface temperature >160 K, and 0 percent saturated pixels from lat 75° S. to 75° N. with unsummed data. The daytime mosaic covers ~95 percent of the planet, with data gaps most common between lat 50° and 70° N. The daytime data are generally indicative of morphology, as temperature variations are domi-nated by surface orientations relative to solar incidence. The THEMIS IR nighttime mosaic covers the region within lat ±60° and has some missing data at the highest latitudes, resulting in about 80 percent total coverage of the planet. Nighttime IR data represent the thermal inertia of the surface materials, showing that brighter materials have cooled less and, thus, are likely composed of coarser-grained materials, indurated surfaces, and (or) bedrock (*, Fergason and others, 2006; Putzig and Mellon, 2007). These data were used to assist in discriminating contacts in regions where the intrinsic thermal inertias of map units were not overly obscured or altered (Christensen and others, 2004).

Initial geologic mapping results using the base maps were verified and honed through integrating other data sets, includ-ing MOLA-derived products (especially color-shaded-relief and slope-detrended maps) and local, higher-resolution images via web-linked image footprints using geographic informa-tion system (GIS) software. Where available, higher-resolution images provided important information regarding unit texture and stratigraphic relations. In some instances, THEMIS visual range images (18 to 36 m/pixel) (Christensen and others, 2004) and Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) images (5 to 6 m/pixel) (Malin and others, 2007) were consulted where THEMIS IR data are lacking or where identifi-cation of small features and textures on scales of tens of meters was required to confirm unit characteristics and to determine and (or) refine contact locations and stratigraphic relations. Other spacecraft data sets (for example, spectral and radar sounding) were not directly used for geologic mapping. Rather, we consulted and implemented published topical investigative results that relied on these types of data, as noted through cita-tion, to help characterize and interpret map units and features and their relations that pertain to global geologic history.

Mapping MethodsPhotogeologic mapping methods that are routinely

employed to identify geologic units on Mars have a heritage in approaches applied to remotely sensed image and topographic data acquired through terrestrial and lunar sensors (*, Wilhelms, 1990; Smith and Pain, 2009). Geomorphic characteristics not only can be recognized in such data, but they generally are more ubiquitous, mappable, and interpretable with respect to the type, timing, and physical modification of bedrock geologic

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materials than are compositional signatures and other data sets. The latter primarily reveal surficial information or very low resolution geophysical data that pertain to buried materials and structures (though all existing information informs on aspects of the geology of Mars). Geomorphic characteristics that are most useful to geologic mapping include stratigraphic relations (for example, layers, layer truncations, and unit embayments and (or) burial), lithologic properties (for example, impact, mass-wasting, and volcanic morphologies), and modificational features (for example, tectonic, fluvial, and periglacial land-forms). In addition, terrestrial photogeologic mapping studies based on simulated planetary data sets have applied ground truth to test the reliability of planetary photogeologic mapping approaches (*, Tanaka and others, 2009; Skinner and Fortezzo, 2013), leading to a conservative and defensible methodology that we employ here. With some minor refinement to defining crater-density boundaries (Werner and Tanaka, 2011; Michael, 2013), the eight-epoch chronology system of Tanaka (1986) is employed in the current map.

Map Development and Author RolesThe methods described in the following subsections chiefly

result from (1) our collective experience mapping Mars (*, Scott and Tanaka, 1986; Tanaka and Scott, 1987; Dohm and others, 2001a; Tanaka and others, 2005; Skinner and Herkenhoff, 2012; Tanaka and Fortezzo, 2012; Irwin and Grant, 2013); (2) the state of the art in handling global mapping data sets using digital mapping technologies (see below); (3) methods used in recent terrestrial geologic maps at global (*, Bouysee, 2009) and conti-nental (*, Reed and others, 2005) scales; (4) guidance provided by the Geologic Mapping Subcommittee of NASA’s Planetary Geology and Geophysics Program and its Planetary Geologic Mapping Handbook (Tanaka and others, 2010); and (5) the application of the interrelated, guiding principles of consistency, simplicity, clarity, utility, and communicability as chosen by our mapping team.

The production of a global map necessitates spatial and categorical generalization of geologic units and features that would otherwise be mapped in more detail at larger scales (*, Varnes, 1974). These generalizations can be accommodated at the selected map scale by carefully balancing cartographic symbols and geologic descriptions. However, unlike many terrestrial examples of small-scale geologic maps (*, Bouysee, 2009), the map presented herein represents original mapping work, augmented with a variety of published topical science investigations, rather than the compilation and generalization of multiple larger-scale geologic map products.

The initial mapping of units and features by region was the responsibility of four of the authors: Tharsis region and Argyre basin (Dohm), northern lowlands and Arabia Terra (Skinner), southern highlands (Irwin), and south polar region and Hellas basin (Kolb). To establish consistency, the global-line-feature mapping and contact attributing were later given to a single mapper (Fortezzo). The entire map was edited and com-piled under the direction of the lead author (Tanaka) through

iterative reviews with primary mappers as well as mapping team discussions, when the schemes for unit naming, grouping, and coloring, as well as for contact and line-feature attributing were developed. Outcrop unit assignments were based partly on age, which required determination of high-accuracy, local crater-density ages of type localities (Platz and Michael). To assist in global unit and outcrop age assignments, we compiled cumulative crater densities for all unit occurrences through a global crater database (Robbins and Hynek, 2012) (see Age Determinations section below). Data management and valida-tion and GIS software oversight (Hare) ensured spatial accuracy and efficiency in mapping.

Unit DelineationMap units identify temporally unique geologic materials

of substantial thickness and extent for portrayal at map scale. Identifying characteristics that establish geologic unique-ness include primary (formational) morphology, IR brightness (daytime or nighttime), and (or) albedo characteristics from visual-range-image data, stratification, relative age, and spatial geologic associations, which are described in the Description of Map Units. Primary characteristics and landforms in this map include lobate scarps (identifying flow boundaries within or at the margin of a unit), layering (presence and thickness of layers), dunes, plains, and shields and cones (indicative of volcanic vents). The primary versus secondary (modificational) nature of some morphologic features is uncertain, and defining units using secondary features was avoided (although they were commonly helpful in establishing time gaps between adjacent units, as well as typical modification history; they are noted in the additional characteristics column in the Description of Map Units). Our unit delineation approach thus differs from the pre-vious global geologic maps of Mars, which included secondary morphologic and albedo features among the principal attributes of map units. For the Noachian units, we did not delineate Noachian crater ejecta within them, as shown in many other Mars geologic maps (*, Scott and Carr, 1978; Scott and Tanaka, 1986), because of the difficulty in many cases of identifying the limit of the ejecta plus the fact that most Noachian materials are interpreted to include impact ejecta.

Units are delineated by relative age as borne out by stratigraphic overlap and embayment relations, and their chronostratigraphic ages were determined by the densities of impact crater populations. Units were assigned ages according to the three periods previously defined by Scott and Carr (1978) and the eight epochs that the periods were later divided into by Tanaka (1986). Many units are likely to consist of complexly intermixed materials of contrasting age and provenance. Such intercalations are neither consistently observable nor possible to represent using regional to global data sets and small map scales. We ignored surficial materials that are estimated to be meters thick (or smaller), including dust, duricrust, desert pavement, and icy soils and mantles (*, Mustard and others, 2001; Putzig and Mellon, 2007). These generally are too thin to recognize and map at global scale. An exception is the approxi-mately meter-thick Amazonian polar cap unit that constitutes

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distinctively bright, mappable surfaces in the polar regions (*, Langevin and others, 2005; Tanaka and Fortezzo, 2012). Because of improvements not only in data resolution and qual-ity but also in mapping approaches and thoroughness of crater dating, many unit exposures have improved age assignments with more consistent accuracy compared to those of the Viking-based global map (Scott and Tanaka, 1986; Greeley and Guest, 1987; Tanaka and Scott, 1987).

Unit GroupsGlobal and continental scale maps of Earth are neces-

sarily made up of broadly inclusive units that can be grouped naturally, given their composition and (or) geographic occur-rence. For example, in the “Geological Map of the World” (Bouysee, 2009), onshore and offshore areas make up the major unit groups, each consisting of lithologic divisions. Also, in the “Geological Map of North America” (Reed and others, 2005), materials are grouped lithologically as sedimentary, volcanic, plutonic, metamorphic, and undivided crystalline rock types. Both of these maps further divide the lowest-rank groups into units according to chronostratigraphic age.

Similarly, we employ a unit grouping scheme that uses both geographic setting and lithologic divisions for unit names and labels. Geographic setting consists of highland, lowland, transitional, basin, polar, and apron groups, all containing mul-tiple units subdivided primarily by age and, in some cases, pri-mary morphologic character. The lithologic units are composed of volcanic and impact (crater) categories. All groups (except the globally distributed impact unit) coincide closely with major geographic zones (see Physiographic Setting), and some groups typically have either low or high kilometer-scale roughness (Kreslavsky and Head, 2000; see Description of Map Units).

In contrast, the previous Viking-based global geologic map included formations and assemblages that, in some cases, distin-guished materials of similar origin and age but were associated with a particular geographic feature that was incorporated into the name. The post-Viking geologic map of the northern plains of Mars (Tanaka and others, 2005) dispensed with formation names for units but still incorporated geographic nomencla-ture in many unit names. Thus, the new map presented here is closer to the approach used in the original, Mariner 9-based map that grouped 24 geomorphic units (lacking formal geographic names) into 5 terrain types.

Unit Names and LabelsUnit names and labels consist of the following compo-

nents: (1) age, (2) unit group, and (3) subtype (optional). For the first time in a map of Mars, the age assignments are included in the unit names to the precision of epochs where possible. This approach is consistent with approaches used in most terrestrial geologic maps (*, Reed and others, 2005; Bouysee, 2009). Moreover, the approach elevates the prominence and statistical integrity of crater-density dating efforts applied to map units, which is one of the most significant advancements resulting from this map. The interpreted age of each geologic unit is

identified in the unit label, where capital letters show the strati-graphic period (N, Noachian; H, Hesperian, A, Amazonian) and epoch is denoted by the use of a preceding lower-case letter (e, Early; m, Middle; l, Late). Where units include more than one epoch within a period, the epochs are not discriminated in the unit names and labels—the Correlation of Map Units (CMU) specifies the interpreted epoch range. In a few cases, observed superposition relations noted in the Description of Map Units (DMU) can be more precisely defined than shown in the CMU, which does not portray temporal relations within epochs. In addition, for units spanning more than one Martian period, the youngest period is followed by the oldest in the unit label and name (for example, HN, Hesperian and Noachian). The unit group designation follows the age component in both unit labels and names (a, apron; b, basin; h, highlands; i, impact; l, lowlands; p, polar; t, transition; v, volcanic).

In some instances, we needed to discriminate particular unit subtypes. Hence, the first letter of a modifying word fol-lows unit group names and labels (c, cap; d, dune; e, edifice; f, field; m, massif; o, outflow; u, undivided). The term, “cap”, represents high-albedo, residual ice sheets that cover parts of the polar plateaus. Dune refers to typical yet diverse (barchan, linear, star, chained, and other complex shapes and patterns) eolian bedforms. Edifice represents topographic features con-sistent with volcanic construction. Field represents groups of relatively small (less than a few tens of kilometers across) vol-canic vents and their contiguous lava plains. Massif represents especially rugged, high-relief, heavily cratered materials that typically form the rims of large impact basins. Outflow rep-resents deposits that occur within—and are genetically linked to—large-scale outflow channels. Undivided units refer to layered sequences that may be hundreds of meters to kilometers thick; layering provides a basis for potential stratigraphic divi-sions in larger-scale mapping, particularly where unconformi-ties and (or) lithologic changes can be identified. For example, the Amazonian polar undivided unit (Apu) is divided into four sequential units at 1:2,000,000 scale (Tanaka and Fortezzo, 2012).

Contact TypesContacts are shown as either certain (solid), approximate

(dashed), or internal (solid with younger (y) and older (o) labels). Certain contacts are considered to be well defined by morpho-logic and (or) IR brightness/albedo boundaries at digital mapping scale (1:5,000,000). Approximate contacts are imprecise due to weakly evidenced, obscured, gradational, and (or) complex unit boundaries. We identified internal contacts within Late Hesperian and Late Amazonian volcanic units (lHv and lAv, respectively), where one flow field overlaps and postdates another, and within some occurrences of unit AHv, where the ejecta of one crater overlaps that of another. In places, high-resolution images were used to augment global MOLA and THEMIS daytime IR base maps for contact mapping. For example, CTX images were locally used to verify and refine the contact between Amazonian and Hesperian transition undivided units (AHtu and Htu), based on recognition of relatively small landforms. Given the mapping

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scale and smoothing applied to the drafted linework, extensive cartographic generalization occurs in which the detailed form of complex contacts evident at larger scale has been subdued or lost at the 1:20,000,000 map scale.

Line Feature TypesOur map includes line features categorized by major

geologic types based on their typical morphologic forms (see Explanation of Map Symbols). The cartographic nature of this global product necessitates balanced spatial representation, rather than thorough identification of morphologic and tectonic features. Major feature types are classified according to preser-vation state (fresh, subdued, partly buried, or buried) and width (greater than or less than 10 km) in the digital map product (see map feature GIS attribute table for width designations).

Wrinkle ridges generally indicate tectonic contraction resulting from deep-seated (several kilometers or more) thrust faulting (*, Golombek and others, 2001). Broad grabens (>10 km wide) indicate deep-seated rifting (*, Hauber and others, 2010), whereas narrow grabens are indicative of either hour-glass-configured normal faults (*, Schultz and others, 2007) or shallow detachment along horizontal mechanical interfaces that may be underlain by dikes or hydrofractures (*, Tanaka and others, 1991; Grosfils and Head, 1994). Outflow channels likely formed as the result of catastrophically erupted, pressur-ized groundwater (*, Baker and others, 1992), whereas narrow valleys likely formed through outburst floods and (or) precipi-tation-driven surface runoff (*, Howard and others, 2005). (See Hynek and others, 2010, for more detailed mapping of valleys.) In some cases, mass flows, glaciers, and lava flows may also be responsible for channel formation, and rilles form a specific channel type that generally narrows downslope and is attrib-uted mainly to thermal erosion (*, Nummedal and Prior, 1981; Lucchitta, 1982; Leverington, 2011). Ridges and scarps, where diagnostic morphologic associations are absent, may result from a variety and combination of geologic processes, including tec-tonic and impact deformation; fluvial, glacial, and mass-wasting erosion; and volcanism (*, Anderson and Anderson, 2010). Spiral troughs are particular erosional features related to erosion and transport of the polar plateaus by insolation and katabatic winds (*, Howard, 1978, 2000; Smith and Holt, 2010). Flow directions are mapped to show major trends of volcanic flows. Crater rims >100 km in diameter are drawn where regionally prominent, but possible buried craters (including quasi-circular depressions of Frey, 2006) suggested by tectonic features and shallow circular depressions are excluded. Some impact craters include circular, partial to complete peak rings within them, which are mapped as ridges (Lyot, Lowell, and Schiaparelli). Locally, pit crater chains occur where subsurface excavation led to collapse, likely related to dilational faulting and fissuring and possibly enhanced locally by groundwater flow (*, Wyrick and others, 2004). Rims of volcanic calderas >100 km across indicate sites of large-scale magma-chamber collapse, associ-ated with effusive flank eruptions and central explosive erup-tions, and are identified on the largest volcanic edifices of Mars (*, Crumpler and others, 1996; Williams and others, 2009).

Drafting

We used the Environmental Systems Research Institute, Inc. (Redlands, CA) ArcGIS software package to co-register and analyze available datasets. Simple Cylindrical versions of MOLA and THEMIS IR mapping bases were created in lossless GeoJPEG2000 format. For consistency and legibility at map scale, we defined drafting parameters within our digital GIS environment. First, the vertex spacing of drafted linework was set at 5 km (4 vertices per millimeter at 1:20,000,000 scale), which provides sufficient detail and curve rounding when a smoothing algorithm is applied to the linework. We drafted linework while viewing the map base data at 1:5,000,000 scale to ensure sufficient accuracy at print scale. The minimum line-feature length is 100 km, and the minimum outcrop dimensions are 40 km wide by 100 km long for legibility. Given differences between drafting and production scales, we avoided mapping narrow outcrops using parameters consistent with our mini-mum width specifications. Minimum spacing between line-work, including both contacts and other line features, is 40 km. However, we exercised some flexibility in the size and spacing cutoffs to represent critical geologic and stratigraphic relations, including some narrow wall rock outcrops of Valles Marineris. Also, we generalized some segmented, en echelon grabens that were too short to map as single segments by mapping them as single collective features to show the occurrence of particular structural fabrics.

Line symbols were adapted from standard drafting conven-tions (Federal Geographic Data Committee, 2006). In general, units within groups were assigned to color families (yellow for the impact unit; greens for lowland units; blues for polar units; bluish greens for basin units; reds, oranges, and purples for volcanic units; and browns, oranges, and gray tones for apron, transition, and highland units). This approach is similar to that used in previous Martian global map-unit color schemes (Scott and Carr, 1978; Scott and Tanaka, 1986).

Age DeterminationsThe relative age of each map unit was determined using

the scheme of three chronostratigraphic periods and eight epoch divisions of the periods that are based on the crater densities of key referent surfaces (table 1; Tanaka, 1986; Werner and Tanaka, 2011). Detailed crater size-frequencies were obtained for representative counting localities for 23 of the 44 map units (table 2 and fig. 1) primarily from HRSC (High Resolution Stereo Camera, onboard the Mars Express spacecraft) data at 12.5 m/pixel and secondarily CTX data at 5 to 6 m/pixel using the GIS extension CraterTools (Kneissl and others, 2011) and the Craterstats software (Michael and Neukum, 2010). Selection of representative counting localities was required, because (1) the total areas and numbers of craters of specific units were too large for complete counts, (2) many of the units exhibited diverse or modified morphologies and other characteristics that made dating areas displaying the most typical morphologies the most desired, (3) parts of unit surfaces have been modified

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by erosion, tectonism, deposition followed by exhumation, and other resurfacing processes that would also result in atypical crater populations (and thus avoided), and (4) some areas had inadequate, high-resolution (HRSC and CTX) image coverage (for a more thorough treatment, see Platz and others, 2013). In some cases, pertinent crater counts that adequately date the units have been published and are cited where the units are described in the Geologic History section for comparison with (or in lieu of) crater counts in this map (tables 2, 3, and D1). For some units having great durations as established by stratigraphic relations, high degrees of post-emplacement crater oblitera-tion, and (or) wide variations in crater densities, detailed crater counts are not helpful in constraining their ages and thus were not obtained.

Model ages derived from the Hartmann and Neukum (2001) chronology function are based on the Ivanov (2001) crater production function. Parts of the crater size-frequency distributions that fit well to the production function over con-tinuous ranges of diameter were used to determine model ages. The Craterstats software performs adjustments that allow for the fitting of multiple resurfacing ages at progressively smaller diameter craters. Thus, many counts resulted in two or more age populations (table 2). Each crater count was tested for spatial randomness to avoid surfaces where crater populations may have been affected by secondary craters or by inhomogeneous resurfacing (Michael and others, 2012).

Because of their widespread occurrence and role in estab-lishing global stratigraphy, we focused much of our attention on the stratigraphic differentiation of Noachian highland unit out-crops, wherein our mapping identified three morphologic-chro-nostratigraphic categories: (1) Early Noachian—highest relief and highest density of impact craters, (2) Middle Noachian—moderate relief and intermediate density of impact craters, and (3) Late Noachian—low local relief and relatively low density of impact craters. Although contacts between these units gener-ally are not sharply defined, most follow topographic breaks where surface landforms become embayed, buried, and (or) markedly subdued. Where counts of all impact craters (includ-ing buried and otherwise modified craters) yielded multiple surface ages, the nature of the unit being dated was specifically assessed to determine the most appropriate age. For example, model age determinations for Early Noachian units were applied to the oldest crater-retention ages, whereas model ages of some younger units were applied to a younger resurfacing age in the crater distribution. This assumption was verified by examin-ing the appearance of the actual craters being counted. In most instances, we confirmed that buried crater populations pre-date the unit. Moreover, we were able to verify that the ages that we had suspected for these and other stratigraphically significant units were generally correct in the representative areas, which verified our stratigraphy. Once we verified that the epoch and period crater-density boundaries of Tanaka (1986) and Werner and Tanaka (2011) were still valid when applied to this new map, we then could use crater counts to verify or reassign the unit designations for otherwise equivocal outcrops.

Given the above analysis, we now define the Early, Middle, and Late Noachian highland units as new referents

for their respective epochs, thus revising those ages based on the previous Viking-based maps of Mars (Tanaka, 1986). We have similarly defined new referent units for the Hesperian and Amazonian epochs (table 1), which for the Hesperian have heritage to previously used referents (Tanaka, 1986). The Early Hesperian Epoch is defined by the Early Hesperian volcanic unit (eHv), which includes materials of volcanic plains in Hesperia Planum on which the Hesperian Period definition was founded (Scott and Carr, 1978). The unit also includes broad volcanic flow fields spread around the Tharsis region, as well as making up Syrtis Major Planum. The Late Hesperian lowland unit (lHl) defines its epoch, following the precedent of Tanaka (1986). A new Early Amazonian referent is the areally small Early Amazonian basin unit (eAb)—the only map unit confined to this epoch in spite of the epoch’s apparent approximately 2-b.y. duration (table 1). Similarly, only the Middle Amazonian lowland unit (mAl) occurs solely within its epoch. Finally, a new referent for the Late Amazonian is the Amazonian polar undivided unit (Apu), which forms ice-rich layered plateaus at both poles (commonly referred to as “polar layered deposits”). Although the base of this unit is poorly defined, it significantly contains a record of recent climate variation on Mars.

Our approach to crater counting was limited to representa-tive locations ranging in size from 4 x 103 km2 to 90 x 103 km2, where all observed craters were counted down to minimum diameters of between 100 to 400 m depending on the area size and surface age (presented in detail by Platz and others, 2013; see also table 2 and fig. 1 showing areas counted). For most of these locations, we determined two model ages (in other cases, one or three). The older age based on the largest craters present was often assigned as unit formation age to ancient highland and thick volcanic units. The reported resurfacing age is also provided to highlight that surfaces experienced post-formation modification through erosion, deposition, deformation, or impact cratering. For younger units that do not completely bury older surfaces, the resurfacing age has been used to infer unit formation ages (for example, at locality 34 in table 2 and fig. 1, the mapped Late Noachian highland unit, lNh, buries an Early Noachian surface). The older ages reported in those cases cor-respond to the ages of underlying units that were partly buried by the young surface of interest. Although craters were counted to minimum diameters between 100 and 400 m, the minimum diameter used to fit the isochrons was generally ≥400 m (in four instances the minimum diameter was between 200 and 350 m).

We also excluded visible clusters, chains, and strewn fields of secondary impact craters from the counting area. Each measured crater size-frequency distribution was also tested for spatial randomness using the procedure of Michael and others (2012) and discarding any that were not sufficiently random (see also Platz and others, 2013).

Remaining outcrops were too large (some exceeding 100 x 103 km2), too numerous, or otherwise problematic for count-ing craters approximately <1 km in diameter, and they required a simplified although less precise crater-dating approach. For these, we applied the global impact crater database that consists of all craters with diameters >1 km (Robbins and Hynek, 2012).

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This database was imported into our GIS project, enabling auto-matic generation of cumulative crater densities for the entire unit areas (including as needed assimilation of superposed unit AHi outcrops), as well as individual occurrences that could be compared with the assigned and calculated boundary densi-ties for craters >1 km (N1), >5 km (N5), and >16 km (N16) in diameter that define epoch boundaries (table 3). This approach required merging the Amazonian and Hesperian impact unit (AHi) into subjacent units, which allowed proper inclusion of the impact craters that define unit AHi. This exercise was performed for all units where unit AHi occurrences were superposed.

Our representative counts of the Middle Noachian highland unit (table 2 and fig. 1; see also Platz and others, 2013) indicate that craters >10 km in diameter have been preserved, validating our application of N(16) values to discriminate most surfaces for their relative age to the upper and lower boundaries of the Middle Noachian. For the Late Noachian/Early Hesperian and Early Hesperian/Late Hesperian boundaries, N(5) was more accurate based on the statistical preservation of craters larger than this diameter. Though most Amazonian surfaces display craters preserved down to 1 km diameter, some potentially Early Amazonian surfaces of unit AHtu are missing craters as large as 5 to 6 km in diameter due to mass wasting and ero-sion. Most outcrops in our initial mapping had an N(1), N(5), or N(16) value that corresponded with the age of the unit to which they were assigned. Where the crater density was in clear disagreement with the initial age assignment, the outcrop was re-assigned to a morphologically similar unit whose age was in agreement with the outcrop (for example, switching a unit designation from eNh to mNh or eHv to lNv based on crater density). This procedure commonly resulted in the merging of adjacent outcrops that had the same unit designation. Some outcrops do not contain crater populations that yield statistically meaningful crater densities. In these instances, direct crater-density dating is not well constrained, although crosscutting and overlap relations with adjacent units helped to establish mean-ingful stratigraphic assignments.

Crater populations for many Late Noachian and younger surfaces include topographically subdued, often rimless craters that likely belong to buried surfaces. Thus, we did not include such craters when deriving crater densities from the global crater data base in order to obtain accurate ages. We recognize, though, that the unit surface age provides only a minimum age constraint, whereas older age constraints come from the ages of surfaces buried or embayed by the unit. In some cases, such as for the Late Hesperian lowland unit (lHl) and the Amazonian and Hesperian volcanic edifice units (Ave and Hve, respec-tively), the oldest parts of the units are buried, which means that the age of initiation of unit emplacement is unconstrained. To show this lack of constraint, the lower edges of the unit boxes have sawtooth-edged age boundaries in the CMU. In addition, some transition units have two major, inseparable components of different age—combinations of Noachian highland mate-rial and Hesperian or Amazonian plains or apron material. The age ranges for these units span the development of both components.

When dating impact craters and basins >150 km in diam-eter, our results are generally consistent with those of Robbins and others (2013), who dated 76 of the 102 features that they identified in this size range. We have dated nearly all of the features using stratigraphic and geomorphic observations (table 4), crater densities (tables 2 and 4), and other work (noted in table 4). The identification, measurement, and dating of 28 craters are considered to be moderately to highly uncertain (these are marked with an asterisk in table 4), and one crater is now considered to be in error (S.J. Robbins, written commun., 2013). Some discrepancies result, particularly for features that we assigned Late Noachian and younger ages, given how the crater extents were mapped and (in our approach) the number of craters identified within and superposed upon the crater ejecta. In addition, Robbins and others (2013) determined ages based on diameters >10 km to fit to absolute-age schemes, whereas we used craters >5 and >16 km in diameter to determine epoch ages. For example, the mapped extent for Galle crater includes the continuous ejecta, whereas Robbins and others (2013) dated only the rim of Galle. We find 7 to 9 superposed craters >5 km diameter (the relative positions of two craters are equivocal), yielding an N(5) density of 43±16 to 55±18 that corresponds most likely to the Early Amazonian (Werner and Tanaka, 2011). The method of Robbins and others (2013) yielded a median age within the Late Noachian. The ejecta of Galle overlie the Hesperian and Noachian basin unit covering Argyre Planitia, which has a distinctly higher density of craters (N(5) = 209±25, indicating a Late Noachian or Early Hesperian age).

Many outcrops, particularly of Middle and Late Amazonian units, could not be accurately dated using craters larger than 1 km. However, in some cases, published crater counts of smaller craters for these units exist and were utilized for our age designa-tions (*, Vaucher and others, 2009; Hauber and others, 2011; Skinner and others, 2012). Other outcrops and units have been eroded, mantled, or otherwise obscured to the point that crater densities are not indicative of unit age. Post-Noachian impact craters are generally identified and mapped based on the rela-tively pristine morphologic expression of their rim and ejecta but are too numerous to date individually. As such, they are grouped into a single Amazonian and Hesperian impact unit (AHi). The vast lava-flow fields comprising most of the central extents of the Tharsis and Elysium rises show ranges of ages and path-ways of individual flows and local flow fields, commonly with no distinct separations of mappable areas of flows of different epochs. In such cases, we map the flows conservatively as the Amazonian and Hesperian volcanic unit (AHv), although previ-ous mapping distinguished multiple flow units based on their mean crater densities (Scott and Tanaka, 1986).

In summary, assignment of relative ages to the map units has been performed to the precision of the eight time-stratigraphic epochs of Mars (Tanaka, 1986; table 1), as shown in the Correlation of Map Units. These assignments are based on (1) stratigraphic relations revealed by the map, as well as (2) crater size-frequency data provided herein and from other sources (table 5). Thus, the robustness of each age assignment varies according to the quality, quantity, and applicability of mapping results coupled with crater statistical data.

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Geologic HistoryThis global map builds upon previous mapping results

and provides new findings and synthesizes other research regarding the geology of Mars, based largely on spacecraft data obtained since the late 1990s. The geologic map includes detailed stratigraphic relations and crater-density statistics that allow for the reconstruction of regional to global geologic events that shaped the surface of Mars. We provide interpreta-tions of the processes that resulted in the present-day land-scape of Mars, given our mapping-based analysis as well as other published work. Processes that have effectively modi-fied geological units through temporally extended or punctu-ated resurfacing have resulted in a geologic and stratigraphic record that is both complex and only partly accessible through mapping.

Noachian PeriodThe Noachian Period includes the oldest observed

geologic activities and resultant units on the surface of Mars, effectively recording rapidly declining impact bombardment from the Early to Late Noachian Epochs (*, Tanaka, 1986; Hartmann and Neukum, 2001). As a consequence of impact cratering and concurrent degradation processes, primary morphologies tens to hundreds of meters in relief and smaller, including tectonic, volcanic, erosional, sedimentary, and impact landforms, were effectively destroyed—especially those that formed during the Early and Middle Noachian Epochs. An exception is strata exposed in eroded Noachian sequences, including areas of the highlands that contain dense arrays of channels, as well as the walls of Valles Marineris (unit Nhu). Valley systems (*, Craddock and Howard, 2002; Fassett and Head, 2008a; Hynek and others, 2010) are locally associated with basin-filling deposits (*, Fassett and Head, 2008b) and suggest the occurrence of distributed, unmapped, thin outcrops of fluvial sediments included in Noachian units. In addition, Noachian highland volcanic edifices (*, Hodges and Moore, 1994; Xiao and others, 2012) suggest that highland materials include variable amounts of volcanic rocks. Noachian units also contain post-Noachian crater ejecta, which generally is included in the units except where we delineated outcrops >100 km across of the Amazonian and Hesperian impact unit (AHi). Larger volcanoes mapped as the Noachian volcanic edifice unit (Nve) display crater ages indicative of multiple stages of activ-ity (Werner, 2009).

The Noachian geologic record in the lowlands is largely unknown, except for a few degraded and fractured Noachian outcrops east of the Elysium rise and at Acidalia Mensa (mapped as the Hesperian and Noachian transition unit, HNt), the arcuate Acheron Fossae structure north of Olympus Mons (mapped as the Late Noachian highland unit, lNh), and unmapped, buried, and degraded crater forms that pre-date the Hesperian plains-forming materials (Frey and others, 2002). The highland-lowland transition zone has been modified due to erosion by liquid water, water-ice, wind, and gravity-driven processes (*, McGill, 2002, 2005;

Tanaka and others, 2005; Dickson and others, 2008; Chuang and Crown, 2009; Fairén and others, 2011; Davila and others, 2013), as well as by endogenic-driven deformation (*, Phillips and others, 2001).

The Noachian is also characterized by the highest mean global erosion rates (Golombek and others, 2006) and forma-tion of extensive valley networks possibly peaking in inten-sity near the end of the period (*, Howard and others, 2005; Irwin and others, 2005; Fassett and Head, 2008a,b; Hoke and Hynek, 2009; Hynek and others, 2010). Large impact and perhaps tectonic landforms that pre-date the Noachian appear to be preserved in the highlands and lowlands (*, Schultz and others, 1982; Nimmo and Tanaka, 2005; Andrews-Hanna and others, 2008), despite later burial and modification (*, Hiesinger and Head, 2000; Frey and others, 2002; Tanaka and others, 2005). Large extensional and contractional tectonic structures, hundreds to thousands of kilometers long, including Phlegra Montes, Tanais Fossae, older structures of Claritas Fossae, and scarp- and ridge-bounded basins in Terra Cimmeria and Terra Sirenum, point to a changing landscape during the Early and Middle Noachian Epochs, prior to the major rise of the Tharsis region (*, Baker and others, 2007).

Noachian geological units typically have a spectral signa-ture that indicates a high abundance of basaltic compositions, including iron- and low-calcium-bearing pyroxenes and oliv-ine, reported to mark modest crustal differentiation (*, Bibring and others, 2005, 2006). Early and Middle Noachian rocks also contain phyllosilicates and chlorite, which are interpreted to have formed through aqueous alteration (*, Poulet and others, 2005; Murchie and others, 2009; Ehlmann and others, 2011; Ody and others, 2012). Chloride salts within Middle Noachian to Early Hesperian highland units occur in some valleys and basins and are thought to be produced by evapora-tion of briny water that carved valley networks (*, Murchie and others, 2009; Osterloo and others, 2010). Change to a more acidic environment fueled by extensive volcanism and volatile release, such as related to incipient Tharsis rise development (*, Dohm and others, 2001b; Phillips and others, 2001), may explain the reduced signature of phyllosilicates beginning in the Middle to Late Noachian in highland and transition units, after which sulfate-rich layered deposits in Meridiani Planum (unit HNhu) formed (*, Bibring and others, 2006; Ehlmann and others, 2011).

The Martian highlands display zones of intense crustal magnetization in Terra Cimmeria and Terra Sirenum. The mag-netization patterns may correspond to some exposed Noachian rock outcrops and structures, possibly indicating dynamic crustal evolution prior to Tharsis rise development (Dohm and others, 2013). Also, magnetization is weak in the vicin-ity of large impact basins, the northern plains, and volcanic regions, indicating that thermal events erased the possibly more widespread magnetization of early crustal rocks (*, Acuña and others, 2001; Connerney and others, 2001; Langlais and others, 2004). Thus, the Martian magnetic field apparently disappeared prior to the majority of the Early Noachian geomorphic and geologic record, including Hellas basin (*, Nimmo and Tanaka, 2005; Frey, 2006).

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Early Noachian Epoch

The oldest crust on Mars consists of Early Noachian materials and inferred, buried pre-Noachian materials associ-ated with topographically subtle basins that may be vestiges of impacts. These materials largely postdate formation of the broad northern lowlands, which comprise the earliest recog-nized geographic feature of the planet and are likely either of impact or tectonic origin (*, Nimmo and Tanaka, 2005; Frey, 2006; Andrews-Hanna, 2008). Our dating of the largest impacts in the global crater database of Robbins and Hynek (2012) indicates that the Early Noachian Epoch included formation of at least 65 impact basins that are >150 km in diameter (table 4; see also Werner, 2008; Robbins and others, 2013). Eighteen of these basins are deeply buried or highly degraded and their identification, diameter, and age have greater uncertainty when compared to other basins. The largest basin is defined by Hellas Planitia (~2,400 km in diameter) and includes the lowest surface elevation on Mars. The Hellas basin, as an impact feature, includes an asymmetric (~5,000 km in diameter northeast-southwest and ~6,000 km in diam-eter northwest-southeast) annulus of terrain elevated between 1 and 2 km above the surrounding highlands, comprising a volume similar to that missing from within the basin (Smith and others, 1999). The interior and southeastern parts of the annulus are made up of massifs and intervening troughs and plains of the Early Noachian highland massif unit, eNhm, whereas remaining high-standing, rugged areas are the Early Noachian highland unit, eNh. The western and northwestern parts of the rim and annulus of Hellas basin are deformed by concentric and oblique ridges (Hellespontes Montes) and troughs (for example, Scylla Scopulus), which may have been the result of a low-angle impact directed southeast (Tanaka and Leonard, 1995). The ~1,160-km-diameter south polar basin is partly outlined by the Promethei Rupes scarp, which forms the extant basin rim and is largely buried by Planum Australe.

Outcrops of unit eNh in the remainder of the highlands are common east of Hellas basin in Terra Cimmeria and Terra Sirenum, corresponding to the zone of strong crustal magnetiza-tion, but are sparse around Argyre basin and in most highlands north of the equator in Arabia, Margaritifer, and Xanthe Terrae and northern Terra Sabaea. Some outcrops of unit eNh in the southern parts of the Tharsis rise contain dense arrays of linear to sinuous troughs interpreted to be narrow grabens and wide rifts, indicating crustal extension (*, Dohm and others, 2001a; Hauber and others, 2010). Other ridges made up of unit eNh trending concentric to the margin of Daedalia Planum may have resulted from crustal contraction due to Syria Planum-centered uplift (*, Banerdt and others, 1992; Schultz and Tanaka, 1994). About two-thirds of mapped Noachian highland edifices (unit Nhe) and volcanic edifices (unit Nve) may be Early Noachian, which attests to early, widespread volcanism with a concentra-tion south of the Tharsis region (Scott and Tanaka, 1981a; Xiao and others, 2012). Small valleys dissect Early Noachian units in places, and some fluvial dissection and mass wasting likely subdued prominent landforms during this epoch. Overall, Early

Noachian materials consist mostly of impact breccias and melts, igneous rocks, and some sedimentary sequences.

Middle Noachian Epoch The Middle Noachian highland unit (mNh) dominates

highland regions wherever the Early Noachian highland unit (eNh) is sparse or absent, including regions that border the highland/lowland transition zone in Arabia Terra and south of Elysium Planitia, as well as those that surround Argyre basin. Unit mNh also forms relatively low lying surfaces adjacent to higher-standing outcrops of unit eNh; commonly a slope break separates these units, wherein narrow (unmapped) valleys incise the steeper parts of the eNh unit. Unit mNh likely consists of a higher proportion of sedimentary and volcanic materials relative to unit eNh, which may reflect climate conditions conducive to precipitation and runoff (Irwin and others, 2013).

The highlands contain at least 15 Middle Noachian impact basins that are >150 km in diameter (table 4). Argyre and Isidis basins are regionally defined by topographic depressions sur-rounded by the Middle Noachian highland massif unit (mNhm). For Argyre basin, the massif unit has a mean outer diameter of ~1,500 km and is surrounded by the Middle Noachian highland unit to a diameter of ~3,000 km. Unlike Hellas basin, this outer zone does not form a distinctly elevated annulus but includes broad, irregular troughs and ridges that are oriented mostly circumferential to the basin and that likely represent structural rings associated with basin formation.

The Isidis basin is located along the highland/lowland transition and displays morphologic characteristics that are different than the similarly sized Hellas and Argyre basins. For example, Isidis basin has a discontinuous rim of the Middle Noachian highland massif unit (mNhm), which extends for a diameter of ~2,100 km and is interspersed with less rugged, lower-relief Middle Noachian highland unit (mNh) and post-Noachian units. Circumferential, mostly narrow troughs encircle the northwest (Nili Fossae) and southeast (Amenthes Fossae) parts of the Isidis basin rim. The troughs likely formed through crustal extension during the Middle Noachian and were reactivated into the Hesperian, given that they locally cut the Early Hesperian transition unit (eHt). Oenotria Scopuli are basin-facing, circumferential scarps that displace and modify the Early Noachian highland unit (eNh) at a radial distance of 1,400 to 1,500 km from the center of Isidis that appear to form a basin-ring structure. Carbonate, rare on the surface of Mars, occurs in a consistent stratigraphic horizon, in places surround-ing Isidis basin, and reflects neutral to alkaline conditions at the time of formation, likely by alteration of an olivine-rich layer (*, Ehlmann and others, 2008); also, diverse assemblages of altered silicate minerals indicative of low-grade metamorphic or hydrothermal aqueous alteration occur in Noachian terrain west of Isidis basin (*, Ehlmann and others, 2009).

Noachian highland and volcanic edifices (units Nhe and Nve, respectively) are fewer in the Middle Noachian than in the Early Noachian, perhaps due to increased global compres-sional stress and crustal thickening, which restricted the ascent of magma away from regional magmatic centers such as Tharsis

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and Elysium (*, Xiao and others, 2012). Embayment by unit lNh shows that the Thaumasia highlands uplifted mostly during the Middle Noachian, and the development of densely spaced, narrow grabens and rifts across the Thaumasia highlands at this time indicates their contemporaneity with the uplift (Dohm and others, 2001a; Hauber and others, 2010).

Late Noachian Epoch Modification of the Martian cratered highlands con-

tinued in the Late Noachian by volcanic, sedimentary, and impact processes, although at overall reduced rates and extents (*, Golombek and others, 2006; Tanaka and others, 2014). These processes resulted in the accumulation of deposits in both inter- and intra-crater basins, mapped herein as the Late Noachian highland unit (lNh). Unit lNh includes ejecta associated with three impact craters that are >150 km across: Becquerel and Green craters and Orcus Patera (table 4). Unit lNh occurs mostly near unit eNh outcrops, as well as along the eastern margins of Thaumasia Planum and Tempe Terra. They also occur locally in Middle Noachian-dominated Arabia Terra, northern Terra Sabaea, other areas along the highland/low-land transition zone, and the annulus of Argyre Planitia. These associations suggest that areas of higher relief experienced enhanced precipitation and erosion, resulting in sedimentation in adjacent topographic depressions (including impact craters) near the end of the Noachian Period (*, Howard and others, 2005; Irwin and others, 2005; Mest and Crown, 2006; Fassett and Head, 2008a,b; Grant and others, 2009; Hynek and others, 2010). In some cases, outflow channels such as Mawrth and Ma’adim Valles were carved by large water discharges resulting from overtopping of highland basins or breakouts from over-pressured aquifers (*, Carr, 1979; Irwin and others, 2004; Irwin and Grant, 2009).

Denudation is indicated along the margins of Thaumasia Planum and Tempe Terra by local terraces, which are interpreted to be eroded strata and mapped as unit Nhu. The deposition of the resulting detritus may have contributed to outcrops of unit lNh. Within Argyre and Hellas Planitiae, basin floors are occupied by the Noachian and Hesperian basin unit (HNb), suggesting infilling by sedimentation and perhaps volcanism (*, Leonard and Tanaka, 2001; Moore and Wilhelms, 2001, 2007; Hiesinger and Head, 2002). Also, the northeastern margin of Hellas basin includes an outcrop of unit lNh resulting from fluvial sedimentation and volcanism that has been dissected and modified by later fluvial activity and other processes (Bleamaster and Crown, 2010).

The layered Hesperian and Noachian highland undivided unit (HNhu) in Meridiani Planum has been directly (though locally) observed by the Opportunity Mars Exploration Rover (*, Squyres and others, 2006). Previous work indicates a Late Noachian to Hesperian age for this outcrop (Hynek and Phillips, 2008), and large craters within and buried by the unit indicate a Middle Noachian age for the underlying surface (that of unit mNh) (locality 24 in table 2). This deposit of unit HNhu is composed of sulfate-rich eolian and fluvial material covered by a hematite-rich erosional lag (*, Grotzinger and others, 2005)

and may result primarily from aeolian deposition followed by groundwater alteration (*, Andrews-Hanna and others, 2007), as well as by fluvial and marine processes (*, Dohm and others, 2009a).

The oldest recognizable volcanic flows and plains-forming deposits on Mars are mapped as the Late Noachian volcanic unit (unit lNv), which occurs in the Tharsis region and Hellas basin. The most extensive outcrop of unit lNv forms a furrowed deposit in Malea Planum that appears to be sourced from circular paterae displaying low rims and depressed centers (*, Leonard and Tanaka, 2001; Plescia, 2004; Williams and others, 2009). The two most distinctive paterae in this region, Amphitrites and Peneus Paterae, are mapped as the Noachian volcanic edifice unit (Nve). Tyrrhenus Mons, northeast of Hellas basin (also mapped as unit Nve), exhibits highly dissected flanks surrounding a central caldera complex and provides evidence for prolonged volcanic activity begin-ning with explosive volcanism emplacing pyroclastic flows in the Late Noachian (*, Greeley and Crown, 1990; Gregg and others, 1998). Apollinaris Mons (units Nve and Hve) occurs along the highland-lowland transition zone and reveals a long history of volcanic, hydrologic, and perhaps hydrothermal activity (*, El Maarry and others, 2012). Several irregularly shaped, unmapped depressions tens of kilometers across of Late Noachian to Early Hesperian age along the transition zone in northern Arabia Terra have been proposed to be volcanic calderas (Michalski and Bleacher, 2013). Outcrops of unit lNv that contain lava-flow lobes occur in Thaumasia Planum, as well as in scattered plains south of Daedalia Planum. Some outcrops of unit lNh north of Olympus Mons (deformed by Acheron Fossae), in southwestern Tempe Terra, and in parts of the Thaumasia highlands may be volcanic given their prox-imity to the Tharsis region, but swarms of grabens and other modificational features may have obscured potential volcanic morphologies.

Late Noachian crustal deformation is confirmed where tectonic structures transect Late Noachian surfaces that are partly buried by Early Hesperian outcrops. These include (1) dense swarms of narrow grabens of Claritas Fossae and in Icaria Planum radial to Syria Planum, likely in response to litho-spheric flexure due to loading by the Tharsis rise (*, Banerdt and others, 1992; Tanaka and Davis, 1988; Anderson and others, 2001); (2) generally west-northwest-trending grabens of Acheron Fossae (Kronberg and others, 2007); and (3) grabens within and resulting from uplift of the Thaumasia highlands (*, Dohm and others, 2001a). Although Late Noachian surfaces are commonly deformed by wrinkle ridges, the cumulative amount of deformation is comparable to that on Early Hesperian surfaces, suggesting that contractional deformation was minor in the Late Noachian. The Valles Marineris troughs began to form during this epoch by rifting, as indicated by crosscutting and stratigraphic relations among rock materials and tectonic structures, particularly along the western margin of Thaumasia Planum (Witbeck and others, 1991; Dohm and others, 2001a, 2009b). Most of these structures relate to development of the Tharsis rise, indicating that this immense feature had become the dominant magmatic and tectonic locus for the planet by the

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end of the Noachian (*, Tanaka and others, 1991; Anderson and others, 2001).

Hesperian PeriodThe Hesperian represents an intermediate period of Mars’

evolutionary chronology that generally preserves much more diverse processes than the Noachian, due to less cumulative impact gardening and reduced mean erosion rates (*, Hartmann and Neukum, 2001; Golombek and others, 2006). Hesperian activity was dominated by extensive volcanism, tectonism, chaos and outflow channel development; northern plains and polar accumulations; and mass wasting and deposition along the highland/lowland transition zone. Generally, resurfac-ing of the Martian cratered highlands was more localized during the Hesperian Period, which included at least four large impacts 150 to 200 km in diameter (Gale, Holden, Lowell, and Bakhuysen). Acidic weathering conditions during the Hesperian led to sulfate enrichment of layered deposits in Valles Marineris and Gale crater (Hesperian transition undivided unit, Htu) and in Schiaparelli, Terby, and other highland craters (Hesperian and Noachian highland undivided unit, HNhu) (*, Bibring and others, 2006). Tharsis-related volcanism and catastrophic outflow flooding were major contributors to resurfacing during the Hesperian Period at local to possibly global scales (*, Hynek and others, 2003; Baker and others, 2007; Dohm and others, 2009a). The overall amount of formation of precipitation-fed runoff valleys and sedimentary deposits was reduced relative to that of the Noachian, leading to just a few local highland exam-ples (*, Grant and Schultz, 1990; Mest and Crown, 2002, 2003; Mangold and others, 2008). Nine occurrences of the Hesperian volcanic edifice unit (Hve) are mapped in the Tharsis, Elysium, and northeast Hellas basin regions, and the southern fan deposit of Apollinaris Mons also is mapped as unit Hve (the remainder of the volcano is made up of unit Nve). Although construction of Martian volcanic edifices appears to be dominated by effu-sive volcanism, evidence for pyroclastic volcanism is apparent at Apollinaris Mons and Hadriacus Mons and in the Elysium region (*, Tanaka and others, 1992: Crown and Greeley, 1993, 2007; El Maarry and others, 2012).

Some of the most pronounced erosion of geologic terrains on Mars took place during the Hesperian. Along the highland/lowland transition zone, complex arrays of remnant Noachian highland-material inliers are surrounded by Hesperian plains-forming mass-wasting deposits (*, Tanaka and others, 2005), including the Hesperian and Noachian transition unit (HNt). Troughs of Valles Marineris and depressions east and north-east of Valles Marineris were the sites of extensive collapse and flood discharges that resulted in (1) the modification of Noachian highland units to form “chaotic terrain” (Sharp, 1973), mapped herein as the Hesperian transition unit (Ht), and (2) the carving of and sedimentation within outflow channels (forming the Hesperian transition outflow unit, Hto) (*, Rotto and Tanaka, 1995; Tanaka, 1997; Chapman and others, 2010). The eastern rim of Hellas basin presents another area of outflow channel erosion (Crown and others, 1992, 2005; Tanaka and Leonard, 1995). In the highland/lowland transition zone south

of the Elysium rise, layered, wind-carved deposits (units Htu and AHtu) may be pyroclastic or other eolian fines (identi-fied as the Medusae Fossae Formation; Scott and Tanaka, 1982, 1986; Hynek and others, 2003; Mandt and others, 2008), perhaps sourced locally (Scott and Tanaka, 1982) from Tharsis volcanoes (Hynek and others, 2003) or from Apollinaris Mons (Kerber and others, 2011)). Locally, unit Htu includes pos-sible aqueous deposits (DiBiase and others, 2013). Watters and others (2007) noted that units Htu and AHtu generally have a low radar dielectric constant, consistent with either highly porous sediment or ice-rich material covered with thin lags. Consistent with our age assignments, Kerber and Head (2010) and Zimbelman and Scheidt (2012) both found that a large pro-portion of these units (although they mapped them differently) were originally formed in the Hesperian.

The oldest mapped outcrops of ice-rich polar deposits span the Hesperian Period. The plains-forming Hesperian polar unit (Hp) and edifice-forming Hesperian polar edifice unit (Hpe) surround Planum Australe and may have been emplaced through cryovolcanic eruption (Tanaka and Kolb, 2001) and (or) from accumulation of a glacial ice sheet (Head and Pratt, 2001). Unit Hp is marked by narrow sinuous ridges that rep-resent either inverted fluvial channels or eskers (*, Head and Pratt, 2001; Tanaka and Kolb, 2001; Ghatan and Head, 2004). Additional polar mounds and depressions of unit Hpe form Scandia Tholi and Scandia Cavi near Planum Boreum and may result from mud volcanism and diapirism (Tanaka and others, 2003, 2011; Skinner and Mazzini, 2009). The basal materials of Planum Boreum comprise a thickly layered mixture of ice and dust (Phillips and others, 2008; Putzig and others, 2009; Tanaka and Fortezzo, 2012) mapped as the Hesperian polar undivided unit (Hpu).

Early Hesperian Epoch

The Early Hesperian volcanic unit (eHv) identifies are-ally extensive lava fields that partly bury the margins and high-standing internal surfaces of the Tharsis rise, as well as Hesperia and Syrtis Major Plana (*, Hiesinger and Head, 2004). The Spirit Mars Exploration Rover demonstrated that the Gusev crater floor is covered by lavas that date to this time (*, Squyres and others, 2004; Greeley and others, 2005). The Early Hesperian highland unit (eHh) is likely made up of a vari-ety of plains-forming rocks and sediments of uncertain origin. Components may include volcanic rocks proximal to major volcanic centers and perhaps interbedded eolian and lacustrine sediments throughout inter-crater highland basins, some that show dissection, including inlet and outlet valleys (*, Grant and Schultz, 1990; Cabrol and Grin, 1999; Goudge and others, 2012). Four unit HNt outcrops close to one another in Terra Sirenum and Terra Cimmeria include dense clusters of degraded mesas and knobs. Plains with scattered knobs mapped as the Early Hesperian transition unit (eHt) occur along the highland/lowland boundary and east of Elysium rise, apparently resulting from mass-wasting and other resurfacing processes (*, Tanaka and others, 2003). Some of the knobs may be juvenile volcanic or mud vents (Skinner and Tanaka, 2007), although the presence

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of buried crater and trough forms in the unit suggests it is domi-nated by unconsolidated sediments rather than by volcanic rock. In eastern Hellas Planitia, the Early Hesperian basin unit (eHb) may have been emplaced and modified by eolian, lacustrine, and (or) volcanic processes (*, Tanaka and Leonard, 1995; Moore and Wilhelms, 2001, 2007; Crown and others, 2005; Bleamaster and Crown, 2010).

Tectonism during the Early Hesperian was dominated over most of Mars by widespread crustal contraction, perhaps related to planetary cooling, which formed arrays of moderate- to high-relief wrinkle ridges in most unit outcrops of this age (units eHh, eHv, eHt, and eHb) (*, Tanaka and others, 1991; Watters, 1993). Wrinkle ridges vary in orientation and commonly follow buried circular impact structures (*, Watters, 1993). The chas-mata of Valles Marineris continued to form during the Early Hesperian (Witbeck and others, 1991) as a result of at least limited crustal extension, perhaps along steeply dipping faults (Andrews-Hanna, 2012). Crustal extension, related to increased regional magmatism, resulted in the formation of graben arrays in volcanic rocks (unit eHv) within and marginal to the Tharsis rise (*, Anderson and others, 2001). Some Early Hesperian rocks are dissected by fluvial channels, including those in southwestern Hesperia Planum, suggesting persistent, volcani-cally driven groundwater discharge toward Hellas Planitia in the Hesperian (*, Squyres and others, 1987; Crown and others, 1992, 2005; Tanaka and Leonard, 1995; Price, 1998).

Late Hesperian Epoch

The Late Hesperian volcanic unit (lHv) covers several areas of the Tharsis region and a small area that extends southwest from Tyrrhenus Mons (Greeley and Crown, 1990); Syria and Solis Plana and Noctis Fossae are the most areally extensive. Unit lHv appears to be dominated by lava flows, and two internal contacts demarcate sub-regions of contrast-ing relative ages, as determined by overlap relations and flow trends. The Late Hesperian volcanic field unit (lHvf) includes many small shields and fissure vents around Syria Planum. Unit lHvf also occurs in central and northeastern parts of the Tharsis region and on the western flank of Elysium Mons (*, Tanaka and others, 1992; Moore, 2001; Hauber and others, 2011). The Amazonian and Hesperian volcanic unit (AHv) includes most lava-flow surfaces that generally slope away from Tharsis, Alba, and Elysium Montes. Although individual flow surface ages are highly variable (*, Platz and Michael, 2011), we note that Late Hesperian flows are common in this unit, except in close prox-imity to the Tharsis Montes, where the flows are Amazonian in age (Scott and Tanaka, 1981b, 1986). Unit lHvf has been modified by extensional tectonism during the Late Hesperian, resulting in narrow grabens oriented both radial and concentric to shield structures, as well as to the elliptical crest of Syria Planum (*, Anderson and others, 2001, 2004).

The Late Hesperian transition unit (lHt) covers many lower-elevation, plains-forming parts of the highland-lowland transition zone, as well as the floor and some flank areas of Valles Marineris and a knobby plain southeast of Hesperia Planum. This unit consists of materials emplaced from erosion,

re-deposition, and mass-wasting of material from adjacent, higher-standing highland outcrops (Witbeck and others, 1991; Tanaka and others, 2003, 2005). The Amazonian and Hesperian transition undivided unit (AHtu) forms the later of the two transition undivided units, which may have been emplaced beginning in the Late Hesperian. This later unit appears to have accumulated after a hiatus, evidenced by inverted fluvial land-forms (*, Burr and others, 2009), wrinkle ridges, and a denser impact crater population on the surface of unit Htu. Unit AHtu may be largely reworked from unit Htu and from itself on a continuous or episodic basis (Kerber and others, 2011). We also include in this unit an outcrop located west of Kasei Valles, sur-rounded by and stratigraphically within unit AHv, that is pitted and possibly layered as evidenced by changes in pit size with levels in the exposure.

Unit lHt and parts of unit lHv contain wrinkle ridges, although these have less relief than adjacent, Early Hesperian surfaces, indicating continuing global contraction after the Early Hesperian but with lower cumulative strain. Dendritic valleys locally occur on unit lHt around Echus Chasma, documenting precipitation-driven overland flow (Mangold and others, 2008). Other evidence for fluvial erosion in the Late Hesperian is sparse.

The termination of the Late Hesperian Epoch is identi-fied herein as the broad-scale cessation of the emplacement of sedimentary plains in the northern lowlands as represented by the Late Hesperian lowlands unit, lHl (Tanaka, 1986; Parker and others, 1989; Tanaka and others, 2003, 2005; Werner and others, 2011). This unit was likely emplaced (and subsequently modified) through a variety of geologic processes and may form complexly intercalated deposits, perhaps dominated by highland debris transported via outflow channels into Chryse Planitia and the lowlands beyond (*, Parker and others, 1989; Baker and others, 1991; Kreslavsky and Head, 2002; Fairén and others, 2003; Buczkowski and Cooke, 2004; Dohm and others, 2009a). Other contributing processes may include mass wasting and fluvial dissection of the highland-lowland transition zone (*, Frey and others, 1988; Maxwell and McGill, 1988; Tanaka and others, 2001, 2003), volcanism, and eolian erosion and deposition. The lHl unit includes scattered, heavily modified materials forming low-relief and perhaps thinly buried knobs and mesas, some of which may pre-date the Late Hesperian. Unit lHl embays unit lHt and other older units, indicating distinct temporal separation. The Late Hesperian basin unit (lHb) makes up most of Hellas Planitia and may have resulted from a combination of volcanic, lacustrine, glacial, and eolian deposition (Kargel and Strom, 1992; Tanaka and Leonard, 1995; Moore and Wilhelms, 2001, 2007).

Amazonian PeriodAlthough the Amazonian may be the longest geologic

period for Mars (table 1), a cold, dry, and oxidizing environ-ment (Bibring and others, 2006), reduced global impact flux (Hartmann and Neukum, 2001), and sporadic—though perhaps more regionally concentrated and episodic—volcanism and fluvial/glacial activity (*, Dohm and others, 2008; Neukum

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and others, 2010; Grant and Wilson, 2011) suggest that rates of material deposition and cumulative amounts of geologic modi-fication in comparison with previous epochs were significantly reduced (Golombek and others, 2006; Tanaka and others, 2014). Accumulation of polar layered deposits and dunes (*, Kolb and Tanaka, 2001; Koutnik and others, 2002), mid-latitude ice-cored lobate debris aprons (*, Pierce and Crown, 2003; Chuang and Crown, 2005; Plaut and others, 2009), and possible lower-lati-tude relict mountain glacial moraines likely relate to obliquity-driven effects on the climate during various episodes within the Amazonian (*, Laskar and others, 2002; Head and others, 2003). Magmatic-driven processes may have also contributed to environmental change (*, Dohm and others, 2008).

The history of polar ice and sediment accumulations is best documented for the Late Amazonian. The Amazonian polar unit (Ap) makes up Cavi Angusti (adjacent to Planum Australe) and is poorly dated due to modification by pits and coalesced depressions that may have resulted from removal of ice by sublimation and cryovolcanism or volcanic activity (Tanaka and Kolb, 2001; Ghatan and others, 2003).

The Amazonian volcanic edifice unit (Ave) consists of the largest shield volcanoes in the Tharsis region—Olympus, Alba, Ascraeus, Pavonis, and Arsia Montes. The ages of the materials buried within these edifices are unknown and may be Hesperian or earlier in age, given that surrounding lava plains of the Tharsis rise date back at least to the Late Noachian (unit lNv). Volcanism during this period accounts for a large portion of unit AHv that surrounds Tharsis, Elysium, and Alba Montes. In contrast, the periphery of Olympus Mons includes several lobes of a rugged, ridged terrain mapped here as the Amazonian apron unit (Aa). These deposits, also known as the Olympus Mons “aureole deposits” (*, Morris, 1982; Morris and Tanaka, 1994), are interpreted to have formed by gravity-spreading of outer parts of the volcano’s shield (*, McGovern and Morgan, 2009). Unit Aa also includes landslides and slumps in Valles Marineris, which likely formed throughout the Amazonian (Quantin and others, 2004). The Amazonian and Hesperian transition unit (AHtu) continued to accumulate during the Amazonian, perhaps through volcanic air-fall deposition and eolian reworking (*, Scott and Tanaka, 1982; Hynek and others, 2003; Mandt and others, 2008; Kerber and others, 2011).

Early Amazonian Epoch

Galle and Lyot impact basins, each ~220 km in diameter and mapped as unit AHi, formed during this epoch. Hummocky flows compose the Amazonian volcanic unit (Av) in two settings. Northwest of Elysium Mons, unit Av flows are dissected by channels and extend >1,400 km from the margin of the Elysium rise into Utopia basin and are interpreted to be volcanically induced debris flows (lahars) that emanated from the rise and subsequently were modified by fluvial activity (*, Christiansen, 1989; Tanaka and others, 1992; Russell and Head, 2003). The Av unit located northwest of Olympus Mons is likely Early to Middle Amazonian lava flows (Tanaka and others, 2005), which appear to originate from beneath the Amazonian apron unit (Aa) and extend 2,000 km from the edifice. Average slopes

are only 0.01° for the Olympus Mons outcrop and 0.05° for the Elysium rise and Utopia basin flows. The latter largely grade into the hummocky, flat Early Amazonian basin unit (eAb) that fills the lowest part of Utopia Planitia. The unit also occurs as a single outcrop in the low-elevation part of Hellas Planitia, which displays layering and reticulate ridge patterns at high resolution and may be the result of deposition in ice-covered lakes and of subsequent deformation (Moore and Wilhelms, 2001, 2007).

Modest tectonism during the Early Amazonian occurred in areas where volcanism and sedimentation appear to have con-tributed to local crustal loading. Northeast of Alba Mons, north-ernmost extensions of densely arrayed narrow grabens dissect the Late Hesperian lowland unit (lHl); the overall fault system that deforms Alba Mons may have resulted from regional rifting due to northwest-southeast extension and local, hotspot-induced buoyant doming of the Alba Mons shield (*, Tanaka, 1990; Cailleau and others, 2005). In addition, gravitational shear stresses resulting from the immense shield may have caused it to spread northward toward the lowlands, prior to building of the summit edifice (Ivanov and Head, 2006). Northwest of Alba Mons, wrinkle ridges oriented circumferential to the edifice may have formed in response to lithospheric loading imposed by growth of the shield and by overall loading of the Tharsis rise (*, Banerdt and others, 1992; Watters, 1993; Head and others, 2002). Wrinkle ridges associated with Utopia and Isidis Planitiae similarly may have resulted from loading by emplace-ment of volcanic, lowland, and basin units, including units AHv, Av, lHl, eAb, and possibly earlier transition units; deformation may have begun in the Hesperian (or Noachian) and continued into the Early Amazonian (Thomson and Head, 2001; Head and others, 2002; Tanaka and others, 2003, 2005).

Middle Amazonian Epoch

This epoch features emplacement of the Middle Amazonian lowland unit (mAl), based on superposition rela-tions and crater counts (Werner and others, 2011; Skinner and others, 2012). This unit formerly covered much of the northern lowlands, but the majority of the unit appears to have been removed (Skinner and others, 2012), accounting for extensive remnant crater-topped mounds and mesas of the unit commonly referred to as “pedestal craters” (*, Kadish and others, 2010). The unit may be mostly dust and ice derived from erosion of the Hesperian polar undivided unit (Hpu); subsequent, gradual erosion of unit mAl led to pedestal crater formation and perhaps re-deposition as the Amazonian polar undivided unit (Apu) (Skinner and others, 2012).

Elsewhere, Middle Amazonian geologic activity included (1) continued but reduced volcanism in the Tharsis and Elysium regions (contributing to units AHv and Av) (Hauber and others, 2011; Platz and Michael, 2011) and perhaps in the highland-lowland transition zone (unit AHtu) (*, Hynek and others, 2003) and (2) apron unit (Aa) development at Olympus Mons and Valles Marineris and along the highland-lowland transition (unit ANa). Evidence for Middle Amazonian tectonism is sparse (*, Anderson and others, 2001).

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Late Amazonian Epoch

This epoch includes substantial coverage of low-lying parts of Amazonis Planitia by sparsely cratered lava flows sourced southwest of Olympus Mons and flow fields south and east of the Elysium rise that erupted from Cerberus Fossae (*, Fuller and Head, 2002; Plescia, 2003; Tanaka and others, 2005; Dohm and others, 2008; Vaucher and others, 2009; Hamilton and others, 2010; Hauber and others, 2011; Platz and Michael, 2011). Part of the Cerberus Fossae flows reach into Amazonis and Arcadia Planitiae, where they overlie older volcanic flows as well as transitional and lowland units. These outcrops, as well as other flows surrounding the base of Olympus Mons and south of Alba Mons, are mapped as the Late Amazonian volca-nic unit (lAv). In Elysium Planitia, unit lAv includes a branch that extends northward along the eastern margin of Elysium Mons (Hamilton and others, 2010; Hamilton, 2013). The Late Amazonian volcanic field unit (lAvf) is composed of groups of small basaltic shields, fissure vents, and emanating lava flows; the unit occurs in central parts of the Tharsis rise and at Cerberus Fossae, commonly along graben swarms (*, Bleacher and others, 2007; Vaucher and others, 2009; Hauber and others, 2011). Formation of narrow grabens and wrinkle ridges during the Late Amazonian is limited to volcanic regions and generally follows older deformation patterns.

The Amazonian polar undivided unit (Apu) was emplaced on the polar plateaus, Planum Boreum and Planum Australe, during the Late Amazonian (Tanaka and Kolb, 2001; Koutnik and others, 2002; Tanaka, 2005; Tanaka and others, 2008; Tanaka and Fortezzo, 2012). These deposits are overlain by residual ices, forming the Late Amazonian polar cap unit (lApc). The south polar ice cap is colder than the north polar ice cap due to its higher elevation and includes carbon dioxide ice overlying water ice (*, Byrne, 2009), as well as a buried, pitted radar-transparent material as much as 300 m thick that is interpreted to be carbon dioxide ice (Phillips and others, 2011). Planum Boreum is surrounded by the most extensive accumula-tions of dune sands on Mars, mapped as the Late Amazonian polar dune unit (lApd) (Tanaka and Fortezzo, 2012); unmapped, smaller patches of dunes occur globally (*, Hayward and others, 2007). The Late Amazonian apron unit (lAa) occurs along the western to northwestern flanks of the Tharsis Montes and Olympus Mons and was mapped in greater detail by Morris and Tanaka (1994), Scott and Zimbelman (1995), and Scott and others (1998). This unit is dominated by ribbed, lobate aprons that are interpreted as drop moraines produced by cold-based glaciers (*, Head and others, 2003).

Major ResultsSeveral advancements in the characterization and analysis of

Martian geology have resulted from this global mapping effort, in part determined by additional studies (see cited references):

1. This map is based mainly on improved topographic and image data not available in the previous global mapping

effort, enabling much more consistent observation of critical morphologic details and stratigraphic relations that have resulted in a very different and more refined map (compare tables 6 and 7). Highland materials make up 44 percent of the surface, volcanic materials 22 per-cent, and other basin, transition, lowland, polar, apron, and impact materials make up the remaining 34 percent (Tanaka and others, 2014).

2. Compilation of stratigraphic relations and crater-density statistics for map units on a global scale have resulted in the most accurate planet-wide dating of Martian geologic materials yet achieved (see also Platz and others, 2013). A revised list of referent map units has been selected to define each of the eight Martian epochs (table 1). For the first time on a geologic map of Mars, chronostratigraphic ages are incorporated in unit names.

3. The surface of Mars is appreciably older overall than previously understood. The Noachian makes up 45 percent of the surface, the Hesperian 29 percent, and the Amazonian 26 percent. (For more details on the char-acter and resurfacing rates of Mars through time, see Tanaka and others, 2014).

4. The Early, Middle, and Late Noachian highland units (eNh, mNh, and lNh, respectively) have statistically distinct ages, and the units occur at progressively lower mean elevations with successively younger age. Crater morphology statistics indicate that Noachian resurfac-ing was spatially non-uniform, long-lived, and gravity driven, which is consistent with volcanism and arid-zone fluvial and aeolian erosion (Irwin and others, 2013).

5. All impact basins >150 km in diameter have been dated and show a dramatically reduced rate of formation over time: >65 for Early Noachian, >15 for Middle Noachian, ~3 for Late Noachian, ~4 for Hesperian, and 2 for Early Amazonian (table 4).

Acknowledgments

Stuart Robbins (University of Colorado, Boulder) gener-ously supplied us with his global digital crater database of Mars (Robbins and Hynek, 2012) prior to its publication. We are indebted to Mike Carr, David Crown, Ernst Hauber, Brian Hynek, Scott Mest, Mike Shand, Mike Smith, and David Williams for constructive reviews. We also benefited from discussions, conference presentations, and publications of many other researchers referenced herein. Most of this work was supported by NASA’s Planetary Geology and Geophysics Program. Work by Platz was funded by the Helmholtz Association through the research alliance “Planetary Evolution and Life” and a German Research Foundation (Deutsche Forschungsgemeinschaft) grant (PL613/1), and efforts by

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Michael by the German Space Agency (Deutsches Zentrum für Luft- und Raumfahrt, Bonn), grant 50QM1001 (High Resolution Stereo Camera on Mars Express), on behalf of the German Federal Ministry of Economics and Technology.

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Xiao, Long, Huang, Jun, Christensen, P.R., and 4 others, 2012, Ancient volcanism and its implication for thermal evo-lution of Mars: Earth and Planetary Science Letters, v. 323–324, p. 9–18 (doi:10.1016/j.epsl.2012.01.027).

Zimbelman, J.R., and Scheidt, S.P., 2012, Hesperian age for western Medusae Fossae Formation, Mars: Science, v. 336, p. 1683 (doi:10.1126/science.1221094).

25

Epoch Reference crater diameter (km)

Cumulative crater density (no. > ref-erence diameter

per 106 km2)

Start of epoch, Ga Referent unitHartmann 2004

iterationNeukum system

Late Amazonian 1 160 0.274 0.328 Apu

Middle Amazonian 1 600 1.03 1.23 mAl

Early Amazonian 1 2,100 3.24 3.37 eAb

Late Hesperian 5 125 3.39 3.61 lHl

Early Hesperian 5 200 3.56 3.71 eHv

Late Noachian 16 100 3.85 3.83 lNh

Middle Noachian 16 200 3.96 3.94 mNh

Early Noachian --- --- --- --- eNh

Table 1. Martian epoch lower boundary ages from Michael (2013) based on the Hartmann (2004 iteration in Hartmann, 2005) and Neukum (Ivanov, 2001; Hartmann and Neukum, 2001) chronology systems derived from reference crater densities of Tanaka (1986) as updated by Werner and Tanaka (2011). Referent units are chosen from this map. ---, no data.

26

(compare to Platz and others, 2013).Table 2. Assigned model ages1 for selected Mars global map units from 48 representative counting localities2 sorted by latitude from north to south

Locality Number2

Unit label

Center coordinates of counting area3

Area (km2)

Age (Ga) Epoch4 Resurfacing graph5

lat °N. long °E. best fit error + error - max. fit min. fit best fit max. fit min. fit1 lHl 71.10 24.78 69,980

3.51 0.10 0.38 3.61 3.13 lH lH eA

3.07 0.16 0.31 3.23 2.76 eA eA eA

2 eHt 49.91 49.50 40,170

3.65 0.04 0.06 3.69 3.59 eH eH lH

3 eAb 47.90 114.00 39,280

3.57 0.07 0.12 3.64 3.45 lH eH lH

1.98 0.31 0.31 2.29 1.67 eA eA eA

4 Av 38.06 128.28 13,810

3.52 0.11 0.49 3.63 3.03 lH eH eA

0.728 0.095 0.095 0.823 0.633 mA mA mA

5 eAb 37.95 111.20 26,100

3.33 0.11 0.26 3.44 3.07 eA lH eA

0.885 0.075 0.075 0.960 0.810 mA mA mA

6 lHl 35.96 330.66 89,930

3.60 0.05 0.08 3.65 3.52 lH eH lH

2.00 0.11 0.11 2.11 1.89 eA eA eA

7 mNh 35.50 47.87 48,850

3.97 0.03 0.03 4.00 3.94 eN eN mN

3.57 0.02 0.02 3.59 3.55 lH lH lH

8 lHt 34.91 166.95 23,130

3.71 0.06 0.10 3.77 3.61 eH lN lH

3.20 0.14 0.35 3.34 2.85 eA eA eA

1.07 0.07 0.07 1.14 1.00 mA mA mA

9 lAv 31.17 193.68 80,780

3.12 0.34 1.70 3.46 1.42 eA lH eA

0.072 0.018 0.018 0.090 0.054 lA lA lA

1.0

0.110.5 2 5 16

14

1.0

0.10.5 1 2 5 16

150

1.0

0.10.5 1 2 5 16

2

1.0

0.10.5 1 2 5 16

290

1.0

0.10.5 1 2 5 16

58

1.0

0.10.5 1 2 5 16

17

1.0

0.10.5 1 2 5 16

210

1.0

0.110.5 2 5 16

43

1.0

0.110.5 2 5 16

62

27

Locality Number2

Unit label

Center coordinates of counting area3

Area (km2)

Age (Ga) Epoch4 Resurfacing graph5

lat °N. long °E. best fit error + error - max. fit min. fit best fit max. fit min. fit10 lHl 29.75 108.01 30,060

30.27 105.85 3.62 0.05 0.07 3.67 3.55 eH eH lH

1.19 0.08 0.08 1.27 1.11 mA eA mA

11 lHl 26.70 106.38 27,410

3.53 0.07 0.13 3.60 3.40 lH lH lH

1.78 0.15 0.15 1.93 1.63 eA eA eA

12 Hto 22.84 316.70 20,360

3.56 0.09 0.21 3.65 3.35 lH eH eA

1.20 0.10 0.10 1.30 1.10 mA eA mA

13 lNh 22.70 16.95 9,684

3.78 0.05 0.07 3.83 3.71 lN lN eH

3.35 0.06 0.10 3.41 3.25 eA lH eA

14 mNh 22.46 32.20 9,668

3.99 0.03 0.04 4.02 3.95 eN eN eN

0.767 0.070 0.070 0.837 0.697 mA mA mA

15 lHt 18.08 117.37 19,190

3.70 0.05 0.07 3.75 3.63 eH lN eH

2.01 0.09 0.09 2.10 1.92 eA eA eA

16 eHt 15.92 311.47 31,990

3.72 0.03 0.04 3.75 3.68 lN lN eH

2.29 0.11 0.11 2.40 2.18 eA eA eA

17 lHv 14.17 237.52 17,230

3.63 0.07 0.13 3.70 3.50 eH eH lH

2.66 0.27 0.31 2.93 2.35 eA eA eA

18 HNt 11.19 161.48 19,700

3.91 0.08 0.18 3.99 3.73 mN eN lN

3.77 0.05 0.07 3.82 3.70 lN lN eH

1.56 0.12 0.12 1.68 1.44 eA eA eA

Table 2. Assigned model ages1 for selected Mars global map units—Continued

1.0

0.110.5 2 5 16

180

1.0

0.110.5 2 5 16

36

1.0

0.110.5 2 5 16

98

1.0

0.110.5 2 5 16

18

1.0

0.110.5 2 5 16

210

1.0

0.110.5 2 5 16

160

1.0

0.110.5 2 5 16

31

1.0

0.110.5 2 5 16

120

1.0

0.110.5 2 5 16

120

28

Locality Number2

Unit label

Center coordinates of counting area3

Area (km2)

Age (Ga) Epoch4 Resurfacing graph5

lat °N. long °E. best fit error + error - max. fit min. fit best fit max. fit min. fit19 mNh 9.52 51.36 43,600

3.94 0.02 0.03 3.96 3.91 mN eN mN

3.69 0.01 0.01 3.70 3.68 eH eH eH

20 mNh 9.43 342.48 22,820

3.91 0.05 0.07 3.96 3.84 mN eN mN

3.34 0.04 0.04 3.38 3.30 eA lH eA

21 lAv 5.41 177.26 45,340

3.37 0.18 1.50 3.55 1.87 eA lH eA

0.014 0.002 0.002 0.016 0.012 lA lA lA

22 mNh 4.39 16.91 32,160

3.93 0.05 0.08 3.98 3.85 mN eN mN

3.68 0.02 0.02 3.70 3.66 eH eH eH

23 mNh 3.98 318.74 26,290

3.89 0.03 0.03 3.92 3.86 mN mN mN

3.55 0.02 0.02 3.57 3.53 lH lH lH

24 HNhu 2.76 355.64 11,680

3.91 0.04 0.05 3.95 3.86 mN eN mN

2.68 0.33 0.40 3.01 2.28 eA eA eA

25 AHtu 1.16 141.90 8,208

3.46 0.06 0.11 3.52 3.35 lH lH eA

26 Htu 0.51 148.66 20,100

3.70 0.04 0.06 3.74 3.64 eH lN eH

1.65 0.15 0.15 1.80 1.50 eA eA eA

27 AHtu -0.01 152.68 57,350

3.50 0.07 0.14 3.57 3.36 lH lH eA

Table 2. Assigned model ages1 for selected Mars global map units—Continued

1.0

0.110.5 2 5 16

190

1.0

0.110.5 2 5 16

100

1.0

0.110.5 2 5 16

66

1.0

0.110.5 2 5 16

170

1.0

0.110.5 2 5 16

93

1.0

0.110.5 2 5 16

170

1.0

0.110.5 2 5 16

57

1.0

0.110.5 2 5 16

150

1.0

0.110.5 2 5 16

17

29

Locality Number2

Unit label

Center coordinates of counting area3

Area (km2)

Age (Ga) Epoch4 Resurfacing graph5

lat °N. long °E. best fit error + error - max. fit min. fit best fit max. fit min. fit28 Htu -4.53 174.64 10,410

3.88 0.08 0.17 3.96 3.71 mN eN lN

2.79 0.27 0.35 3.06 2.44 eA eA eA

29 lHv -7.50 252.99 6,508

3.55 0.09 0.28 3.64 3.27 lH eH eA

1.59 0.20 0.20 1.79 1.39 eA eA eA

30 AHtu -8.12 182.28 23,890

3.49 0.15 2.10 3.64 1.39 lH eH eA

0.231 0.032 0.032 0.263 0.199 lA lA lA

31 mNh -8.50 345.53 21,190

3.87 0.04 0.05 3.91 3.82 mN mN lN

3.50 0.03 0.03 3.53 3.47 lH lH lH

32 eNh -14.05 49.41 19,900

3.99 0.04 0.05 4.03 3.94 eN eN mN

3.50 0.02 0.03 3.52 3.47 lH lH lH

33 eNh -15.00 82.62 21,530

3.99 0.03 0.04 4.02 3.95 eN eN eN

3.50 0.02 0.03 3.52 3.47 lH lH lH

34 lNh -17.74 330.63 39,000

4.08 0.07 0.13 4.15 3.95 eN eN eN

3.79 0.02 0.02 3.81 3.77 lN lN lN

35 lNh -21.85 354.96 35,180

3.72 0.06 0.09 3.78 3.63 lN lN eH

3.36 0.04 0.06 3.40 3.30 eA lH eA

36 eNh -23.57 35.53 42,060

4.03 0.05 0.07 4.08 3.96 eN eN eN

3.82 0.02 0.03 3.84 3.79 lN mN lN

3.66 0.01 0.01 3.67 3.65 eH eH eH

Table 2. Assigned model ages1 for selected Mars global map units—Continued

1.0

0.110.5 2 5 16

14

1.0

0.110.5 2 5 16

460

1.0

0.110.5 2 5 16

74

1.0

0.110.5 2 5 16

47

1.0

0.110.5 2 5 16

270

1.0

0.110.5 2 5 16

280

1.0

0.110.5 2 5 16

120

1.0

0.110.5 2 5 16

49

1.0

0.110.5 2 5 16

260

30

Locality Number2

Unit label

Center coordinates of counting area3

Area (km2)

Age (Ga) Epoch4 Resurfacing graph5

lat °N. long °E. best fit error + error - max. fit min. fit best fit max. fit min. fit37 Nhu -27.04 288.95 19,210

3.73 0.02 0.03 3.75 3.70 lN lN eH

38 eHv -28.26 117.58 40,740

3.65 0.04 0.05 3.69 3.60 eH eH lH

3.01 0.05 0.05 3.06 2.96 eA eA eA

39 lHb -32.07 65.62 8,412

3.02 0.35 1.10 3.37 1.92 eA eA eA

0.394 0.036 0.036 0.430 0.358 mA mA mA

40 HNb -32.78 62.82 74,590

-33.54 59.73 3.70 0.04 0.05 3.74 3.65 eH lN eH

-35.48 55.30 1.96 0.15 0.15 2.11 1.81 eA eA eA

41 eAb -36.83 52.81 4,172

-38.88 51.92 3.11 0.28 1.10 3.39 2.01 eA lH eA

0.098 0.011 0.011 0.109 0.087 lA lA lA

42 eHv -41.92 254.04 7,631

3.59 0.08 0.16 3.67 3.43 lH eH lH

0.935 0.073 0.073 1.01 0.862 mA mA mA

43 eNhm -45.19 43.33 53,860

3.99 0.03 0.04 4.02 3.95 eN eN eN

3.87 0.02 0.02 3.89 3.85 mN mN mN

3.46 0.02 0.03 3.48 3.43 lH lH lH

44 eHb -45.99 86.77 36,310

3.78 0.03 0.04 3.81 3.74 lN lN lN

3.34 0.05 0.07 3.39 3.27 eA lH eA

45 eNhm -52.52 106.74 26,810

4.11 0.06 0.11 4.17 4.00 eN eN eN

3.31 0.09 0.18 3.40 3.13 eA lH eA

Table 2. Assigned model ages1 for selected Mars global map units—Continued

1.0

0.110.5 2 5 16

41

1.0

0.110.5 2 5 16

15

1.0

0.110.5 2 5 16

1.1

1.0

0.110.5 2 5 16

39

1.0

0.110.5 2 5 16

240

1.0

0.110.5 2 5 16

520

1.0

0.110.5 2 5 16

250

1.0

0.110.5 2 5 16

270

1.0

0.110.5 2 5 16

280

31

Locality Number2

Unit label

Center coordinates of counting area3

Area (km2)

Age (Ga) Epoch4 Resurfacing graph5

lat °N. long °E. best fit error + error - max. fit min. fit best fit max. fit min. fit46 lNv -56.66 68.40 60,380

3.89 0.08 0.21 3.97 3.68 mN eN eH

3.67 0.04 0.05 3.71 3.62 eH eH eH

47 lNv -57.05 35.18 26,420

3.78 0.05 0.08 3.83 3.70 lN lN eH

2.54 0.28 0.30 2.82 2.24 eA eA eA

48 Hp -73.21 301.28 33,160

3.76 0.02 0.03 3.78 3.74 lN lN lN

3.57 0.01 0.01 3.58 3.56 lH lH lH1Crater size-frequency distribution model absolute age and Martian epoch fits using production function of Ivanov (2001) and epoch boundaries in the Neukum chronology

system as defined by Michael (2013) (see table 1 for epoch boundaries, Age Determinations section for discussion, and Platz and others (2013) for detailed analysis). Each age fit is shown on separate rows, where the oldest fit reflects the age of the unit in most cases (except for crater localities 3, 9, 15, 21, 34, 39, 44, 46, and 48). For locality 18, the first two age fits apply to unit HNt, which is a composite unit.

2Crater counting localities are documented in figure 1.3Center coordinates of counting area; multiple coordinates indicate multiple subareas.4N, Noachian; H, Hesperian; A, Amazonian; e, Early; m, Middle; l, Late.5Resurfacing graphs give an at-a-glance impression of both crater density and resurfacing history (crater loss at lesser diameters) (see Platz and others, 2013). They show

cumulative crater density per million square kilometers (gray digits, rounded to two significant figures) at a given reference diameter (heavy black mark on axis) and plot rela-tive crater density in a given reference diameter interval with respect to the predicted value, based on the observed density at the next larger reference diameter (assuming the Ivanov, 2001, production function). Vertical axis shows the degree of resurfacing at a given diameter, where a value of 1.0 indicates no resurfacing, and a value of 0.1 indicates that 90 percent of the craters have been erased. The value is the ratio of the observed number of craters in the diameter interval to the number predicted from the observed num-ber in the next larger interval using the Ivanov (2001) production function (Platz and others, 2013).

Table 2. Assigned model ages1 for selected Mars global map units—Continued

1.0

0.110.5 2 5 16

33

1.0

0.110.5 2 5 16

69

1.0

0.110.5 2 5 16

7.5

32

Unit name Unit label

Area (106 km2)

N(1) Error N(2) Error N(5) Error N(16) Error

LOWLAND UNITS

Middle Amazonian lowland unit

mAl 3.13 1,544.9 22.2 489.5 12.5 121.0 6.2 23.3 2.7

Late Hesperian lowland unit lHl 17.28 1,573.5 9.5 512.4 5.4 109.7 2.5 23.8 1.2

POLAR UNITS

Late Amazonian polar cap unit

lApc 0.70 14.3 4.5 11.5 4.1 10.0 3.8 7.2 3.2

Late Amazonian polar dunes unit

lApd 0.30 163.4 23.3 103.4 18.6 46.7 12.5 3.3 3.3

Amazonian polar undivided unit

Apu 2.00 271.3 11.7 143.6 8.5 76.6 6.2 32.0 4.0

Amazonian polar unit Ap 0.22 808.2 60.6 254.3 34.0 113.5 22.7 22.7 10.2

Hesperian polar unit Hp 1.35 3378.2 50.0 997.4 27.2 233.6 13.1 62.1 6.8

Hesperian polar undivided unit

Hpu 0.03 465.5 116.4 436.4 112.7 87.3 50.4 0.0 0.0

Hesperian polar edifice unit Hpe 0.28 795.4 53.6 412.2 38.6 162.7 24.3 32.5 10.8

BASIN UNITS

Early Amazonian basin unit eAb 0.54 1,286.6 48.7 309.2 23.9 75.5 11.8 9.2 4.1

Late Hesperian basin unit lHb 0.92 939.2 32.0 415.2 21.2 132.6 12.0 20.7 4.7

Early Hesperian basin unit eHb 0.42 1,649.2 62.6 535.5 35.7 188.0 21.2 38.1 9.5

Hesperian and Noachian basin unit

HNb 0.66 1,361.1 45.5 583.3 29.8 203.6 17.6 53.2 9.0

VOLCANIC UNITS

Late Amazonian volcanic unit

lAv 3.43 551.8 12.7 203.9 7.7 72.5 4.6 19.5 2.4

Late Amazonian volcanic field unit

lAvf 0.31 192.7 24.9 96.4 17.6 35.3 10.7 9.6 5.6

Amazonian volcanic unit Av 2.16 772.6 18.9 255.8 10.9 74.5 5.9 7.4 1.9

Amazonian and Hesperian volcanic unit

AHv 13.33 1,303.3 9.9 339.1 5.0 85.2 2.5 16.4 1.1

Late Hesperian volcanic unit lHv 2.47 2171.5 29.7 509.7 14.4 98.4 6.3 15.4 2.5

Late Hesperian volcanic field unit

lHvf 0.44 1670.2 61.6 402.18 30.3 111.5 15.9 15.9 6.0

Early Hesperian volcanic unit

eHv 6.24 2,960.1 21.8 820.3 11.5 246.7 6.3 73.4 3.4

Late Noachian volcanic unit lNv 2.45 2,213.9 30.1 804.8 18.1 258.7 10.3 74.3 5.5

Amazonian volcanic edifice unit

Ave 0.82 457.7 23.6 97.4 10.9 15.8 4.4 0.0 0.0

Hesperian volcanic edifice unit

Hve 0.38 2,230.0 76.4 568.0 38.6 175.4 21.4 28.8 8.7

Noachian volcanic edifice unit

Nve 0.21 2,658.4 113.7 845.6 64.1 330.5 40.1 34.0 12.9

APRON UNITS

Late Amazonian apron unit lAa 0.28 158.0 23.6 63.2 14.9 14.0 7.0 0.0 0.0

Table 3. Total crater densities for map units on Mars, based on intersection of crater center points with unit outcrops.

[Crater data are from Robbins and Hynek (2012). N(x) = number of craters > x km in diameter/counting area (in units of 106 km2). Error = (number of craters > x km in diameter) 0.5 / counting area (in units of 106 km2). The impact unit is not included but has been assimilated into underlying map units to more accurately and completely measure their crater densities. See Age Determinations section and table 5 for how these data relate to unit ages]

33

Unit name Unit label

Area (106 km2)

N(1) Error N(2) Error N(5) Error N(16) Error

APRON UNITS—continued

Amazonian apron unit Aa 0.99 130.7 11.5 71.4 8.5 18.1 4.3 2.0 1.4

Amazonian and Noachian apron unit

ANa 0.38 1,017.7 51.5 481.5 35.4 195.2 22.2 44.2 10.7

TRANSITION UNITS

Amazonian and Hesperian transition undivided unit

AHtu 2.23 614.5 16.6 230.7 10.2 62.8 5.3 6.7 1.7

Hesperian transition undi-vided unit

Htu 0.75 2,262.8 54.8 608.4 28.4 176.3 15.3 37.1 7.0

Late Hesperian transition unit

lHt 2.40 1,950.2 28.5 549.7 15.1 160.6 8.2 33.3 3.7

Early Hesperian transition unit

eHt 3.95 2,715.1 26.2 848.2 14.7 238.7 7.8 50.9 3.6

Hesperian transition unit Ht 0.91 1,191.6 36.2 479.5 22.9 184.3 14.2 66.9 8.6

Hesperian transition outflow unit

Hto 1.35 1,868.6 37.1 481.4 18.9 130.7 9.8 32.5 4.9

Hesperian and Noachian transition unit

HNt 3.15 2,557.4 28.5 906.7 17.0 328.0 10.2 99.6 5.6

HIGHLAND UNITS

Hesperian and Noachian highland undivided unit

HNhu 1.07 3,125.4 54.1 1185.5 33.3 379.5 18.9 109.6 10.1

Noachian highland undivided unit

Nhu 1.65 1,667.3 31.7 504.1 17.4 148.5 9.5 39.2 4.9

Early Hesperian highland unit

eHh 1.89 2,730.9 38.0 727.6 19.6 181.8 9.8 32.3 4.1

Late Noachian highland unit lNh 8.98 3,646.9 20.1 1183.8 11.5 385.3 6.5 116.0 3.6

Middle Noachian highland unit

mNh 34.80 4,172.3 11.0 1487.1 6.5 581.1 4.1 177.4 2.3

Early Noachian highland unit eNh 15.33 3,877.3 15.9 1481.7 9.8 665.4 6.6 249.2 4.0

Noachian highland edifice unit

Nhe 0.22 3,894.6 132.9 1459.9 81.4 702.8 56.4 172.3 27.9

Middle Noachian highland massif unit

mNhm 1.97 2,669.1 36.8 1134.5 24.0 468.7 15.4 147.3 8.6

Early Noachian highland massif unit

eNhm 1.98 2,707.9 37.0 1231.9 24.9 598.3 17.4 217.9 10.5

Table 3. Total crater densities for map units on Mars, based on intersection of crater center points with unit outcrops.—Continued

34

Table 4. Impact craters and basins (and possible impact-like features of other origin) on Mars >150 km in diameter totaling 101 in number, from largest to smallest, as identified by Robbins and others (2013).

[One feature in their database located at lat 29.9° S., long 168.6° E. is an error (S.J. Robbins, personal commun., 2013) and is not included in this table. Crater names are from the International Astronomical Union where available (see http://planetarynames.wr.usgs.gov/); for some unnamed craters (---), the name of a chaos feature within the crater is indicated in parentheses. Crater ages are chronostratigraphic period and epoch designations: N, Noachian; H, Hesperian; e, Early; m, Middle; l, Late. Age basis provides the primary basis for the age assignments, including crater counts in table 2, Werner (2008), and Irwin and others (2013); stratigraphic relations; and crater densities, where N(5 or 16) equals the number of superposed craters ≥5 or 16 km diameter per 106 km2 using the database of Robbins and Hynek (2012). The unit(s) column shows how we mapped preserved and, in some cases, overlying surfaces of the impact features. See text for further discussion]

Crater name Diameter (km) Latitude (°N.) Longitude (°E.) Crater age Age basis Unit(s)

Hellas basin ~2,400 –42 71 eN table 2 eNhm

Isidis basin ~1,500 13 89 mN N(16)=185±20 mNhm

Argyre basin ~900 –50 318 mN N(16)=133±10 mNhm

Huygens 467 –13.9 55.6 eN Werner (2008) eNh

Schiaparelli 446 –2.5 16.8 mN N(16)=187±55 mNh

--- 427 –36.8 2.8 eN N(16)=302±47 eNh

Cassini 408 23.4 32.1 eN N(16)=213±38 eNh

Antoniadi 401 21.4 60.8 eN superposed by Baldet

eNh

---* 376 36.7 192.4 ~N map unit HNt

Dollfus 359 –21.6 356.2 eN N(16)=351±88 eNh, mNh

Tikhonravov 344 13.3 35.9 eN N(16)=334±54 eNh

---* 341 –58.8 283.2 eN/mN map unit mNh

---* 340 23.5 53.2 eN map unit eNh

---* 327 –52.7 250.5 eN map units eNh, mNh, AHi

--- 326 –0.4 28.9 eN N(16)=229±45 eNh, mNh, AHi

Newton 312 –40.4 201.9 eN N(16)=212±57 eNh, mNh

de Vaucouleurs 312 –13.3 171.1 mN Werner (2008) eNh, mNh

--- 302 –59.9 275.9 eN N(16)=235±65 eNh, mNh

Copernicus 302 –48.8 191.2 eN Werner (2008) eNh

Herschel 298 –14.5 129.9 mN N(16)=154±40 mNh

Schroeter 292 –1.9 56.0 mN N(16)=119±38 mNh

Koval’sky 285 –29.6 218.6 eN map unit eNh

---* 285 12.9 41.9 eN buried mNh

--- 279 54.4 82.0 H/N buried lHl

--- (Aram Chaos) 276 2.8 338.8 mN N(16)=196±49 mNh

Orcus Patera 263 14.2 178.6 lN N(5)=343±83 lNh

---* 261 –4.3 333.1 N buried Ht, mNh

--- (Atlantis Chaos)

261 –34.4 182.4 eN N(16)=299±86 eNh

--- 257 –58.5 265.5 eN N(16)=224±56; degraded

eNh, mNh

Newcomb 256 –24.2 1.1 eN N(16)=229±57 eNh

---* (Gorgonum Chaos)

250 –37.7 189.3 eN degraded eNh, mNh

Flaugergues 236 –16.8 19.2 mN map unit mNh

--- 229 –50.1 243.1 eN N(16)=282±92 eNh, AHi

35

Crater name Diameter (km) Latitude (°N.) Longitude (°E.) Crater age Age basis Unit(s)

Galle 223 –50.6 329.1 eA N(5)=43±16 to 55±18

AHi

Kepler 222 –46.7 141.2 eN N(16)=285±62 eNh

Lyot 220 50.5 29.3 eA Werner (2008) AHi

---* 218 43.7 18.4 N buried, degraded eHt, lNh, ANa

Secchi 217 –57.8 102.0 mN N(16)=103±46 mNh

---* 215 11.7 0.8 eN buried eNh, mNh

---* 211 –41.8 130.9 eN degraded eNh, mNh

Vinogradov 210 –19.8 322.3 eN N(16)=253±64 eNh

---* 204 –42.8 215.0 eN degraded eNh, mNh

---* 202 47.9 89.8 N/H buried lHl

Kaiser 202 –46.2 19.1 eN N(16)=339±71 eNh

Schmidt 201 –72.0 281.9 eN N(16)=226±100 eNh

--- 199 –71.1 314.2 eN N(16)=331±162 eNh

Lowell 199 –52.0 278.6 eH Werner (2008) AHi

Schöner 199 19.9 50.7 eN N(16)=436±115 eNh, mNh

Mutch 199 0.6 304.8 mN N(16)=159±88 mNh

---* 199 –9.1 20.5 eN degraded eNh, mNh

---* 197 –38.2 197.5 eN degraded eNh, mNh

---* 197 –21.1 217.0 eN degraded, buried eNh, AHv

---* 195 –31.1 293.7 mN? degraded, buried mNh, lNh

---* 195 41.8 21.9 mN? degraded, buried mNh, eHt, ANa

---* 194 –41.6 237.4 eN degraded, de-formed

eNh

---* 190 31.1 186.8 N degraded, buried HNt, lAv

--- 189 –48.3 196.7 eN N(16)=472±193 eNh, mNh

Phillips 185 –66.3 315.2 mN map unit mNh

Dawes 185 –9.1 38.1 eN map unit eNh

Green 182 –52.3 351.5 lN N(5)=270±75 lNh

Baldet 181 22.8 65.5 mN N(16)=209±104 mNh, AHi

---* 180 –39.3 76.8 N buried eHb, lHb

Savich 179 –27.5 96.1 eN N(16)=655±258 eNhm, mNh, eHv, AHi

---* 179 –11.5 356.0 eN N(16)=477±229; buried

mNh

---* 177 –37.5 19.1 eN degraded eNh, mNh

Darwin 176 –57.0 341.0 eN N(16)=241±73 eNh

Molesworth 175 –27.5 149.2 eN N(16)=403±212 eNh

Wallace 175 –52.4 110.9 eN N(16)=241±141 eNhm

--- 175 –23.1 8.5 eN N(16)=627±189 eNh, mNh

Flammarion 174 25.2 48.3 eN N(16)=209±106 eNh, mNh

Terby 171 –28 74.1 eN N(16)=426±219 eNhm, mNh, lNh

---* 171 –23.7 30.4 eN degraded eNh, AHi

Table 4. Impact craters and basins (and possible impact-like features of other origin) on Mars >150 km in diameter totaling 101 in number, from largest to smallest, as identified by Robbins and others (2013).—Continued

36

Table 4. Impact craters and basins (and possible impact-like features of other origin) on Mars >150 km in diameter totaling 101 in number, from largest to smallest, as identified by Robbins and others (2013).—Continued

Crater name Diameter (km) Latitude (°N.) Longitude (°E.) Crater age Age basis Unit(s)

--- 169 –63.2 167.9 eN N(16)=620±235 eNh, lNh

Henry 168 10.8 23.4 eN N(16)=248±157 eNh

Proctor 167 –47.6 29.7 eN N(16)=409±262 eNh, mNh, lNh

---* 167 –23.2 219.7 eN degraded, buried eNh, Nve

--- 166 –18.2 1.6 eN N(16)=259±136 eNh, mNh

---* 166 1.8 125.4 eN/mN degraded mNh, HNt

Becquerel 165 21.9 352.1 lN N(5)=295±85 lNh

Ptolemaeus 165 –45.9 202.3 eN N(16)=256±148 eNh

--- 164 3.2 308.0 eN N(16)=284±128 eNh, AHi

Stoney 161 –69.6 221.6 eN N(16)=376±213 eNh

Denning 160 –17.4 33.5 eN N(16)=483±141 eNh, AHi

--- 159 30.6 9.1 eN N(16)=374±205 eNh

Schaeberle 159 –24.4 50.2 eN N(16)=439±235 eNh

---* 158 –26.8 193.0 eN degraded eNh, mNh

Gusev 158 –14.5 175.5 eN N(16)=899±345 eNh, mNh

Mariner 157 –34.7 195.8 eN N(16)=587±254 eNh

Lohse 155 –43.2 343.4 mN N(16)=104±81 mNh

Gale 154 –5.4 137.8 eH N(5)=173±45 AHi

--- 154 –51.5 231.5 eN N(16)=405±159 eNh

--- 154 –20.2 191.9 eN N(16)=598±241 eNh, mNh

Janssen 154 2.7 37.6 mN N(16)=117±84 mNh

Bakhuysen 153 –23.0 15.8 eH N(5)=151±36 AHi

---* 153 –54.7 244.6 eN buried eNh, mNh, lNh

Holden 153 –26.0 326.0 eH/lH Irwin and Grant (2013)

AHi

Arago 152 10.2 29.9 eN N(16)=455±254 eNh, mNh, AHi

Dejnev 152 –25.1 195.4 eN N(16)=253±123 eNh, AHi

---* 151 6.8 182.2 N degraded, buried HNt, lHt

--- 151 –9.0 353.1 mN map unit mNh

* The impact origin of these features is considered to be uncertain due to poorly preserved or inferred buried rims.

37

Table 5. Basis for time-stratigraphic assignments for each map unit as shown in the Correlation of Map Units.

[Key stratigraphic relations (<, younger than; ~, coeval with; >, older than) are derived from mapping observations (see Description of Map Units for more detail). Crater size-frequency data are sourced from tables 2 to 4 and from within literature references (however, not all units have useful crater size-frequency data). Local-ity numbers refer to specific crater size-frequency data in table 2. N(x), where x = 1, 5, or 16, refers to specific crater-density values in table 3. ---, no data]

Unit label Key stratigraphic relations Crater size-frequency data

LOWLAND UNITS

mAl < lHl; > lApd, most of Apu Werner and others (2011); Skinner and others (2012)

lHl < lHt, Hto; > eAb, Av Localities 2, 6, 10, and 11; N(5); Werner and others (2011)

IMPACT UNIT

AHi < lNh ---

POLAR UNITS

lApc < Apu ---

lApd < mAl, most of Apu ---

Apu < lHl, Hpu, most of Ap; > Apu Koutnik and others (2002); Tanaka (2005); Tanaka and others (2008); Banks and others (2010)

Ap < Hp; > most of Apu N(1); Tanaka and Kolb (2001)

Hp < lNh; > Ap, Apu Locality 48; N(5)

Hpu > Apu Tanaka and Fortezzo (2012)

Hpe ~ Hp, lHl; > Apu, mAl N(5)

BASIN UNITS

eAb < lHb, lHl; ~ Av; > mAl Localities 3, 5, 41; Tanaka and others (2005)

lHb < eHb; > eAb Locality 39; N(5)

eHb < lNb; > lHb Locality 44; N(5)

HNb < mNh; ~ lNv; > eHb Locality 40; N(5) and N(16)

VOLCANIC UNITS

lAv < Av; ~ lAvf Localities 9 and 21; Tanaka and others (2005); Vaucher and others (2009); Hauber and others (2011)

lAvf ~ lAv Tanaka and others (2005); Vaucher and others (2009); Hauber and others (2011)

Av < lHl; ~ eAb; > mAl, lAv Locality 4; Tanaka and others (2005)

AHv < lHl, Hve; ~ eHv, lHv, eAb, lAvf; > Av, mAl, lAa, lAv

N(1) and N(5); Tanaka and others (2005); Hauber and others (2011)

lHv < eHv, eHh; > AHv, AHtu, Aa Localities 17 and 29; N(5); Dohm and others (2001a)

lHvf < eHh; ~ lHv N(5); Hauber and others (2011)

eHv < lNv, lNh; ~ eHt, eHh; > lHv, lHt, lHl Localities 38 and 42; Dohm and others (2001a)

lNv < mNh; ~lNh; > eHv, eHh, eHb Localities 46 and 47; N(5) and N(16); Tanaka and Leonard (1995); Dohm and others (2001a)

Ave ~ lAvf, AHv; > lAa N(1)

Hve < eNhm; ~ lHv, lHvf, lHt, lHl, eHv; > AHv N(5); Tanaka and others (1992); Tanaka and Leonard (1995); Crown and Greeley (2007); El Maary and others (2012)

Nve ~ lNv, mNh, eNh; > eHv, Htu N(5); Greeley and Crown (1992), Gregg and others (1998); Werner (2009); Xiao and others (2012)

APRON UNITS

lAa > Aa, Ave ---

Aa < Av, lHt, lHv; > lAv, lAvf, lAa N(1); Quantin and others (2004)

ANa Noachian part: ~ HNt, lNh, mNh, eNhm. Amazonian part: < AHv, lHt

N(1) and N(16)

38

Unit label Key stratigraphic relations Crater size-frequency data

TRANSITION UNITS

AHtu < Aa, Ave, lHv, eHv; ~ AHv; > lAv Localities 25, 27, and 30); N(5); Tanaka and others (2005); Kerber and Head (2010); Zimbelman and Scheidt (2012)

Htu < Nhu, Nve; ~ eHt, Hve; > lHt, Ht, Hto, AHtu Localities 26 and 28; N(5); Kerber and Head (2010); Zimbelman and Scheidt (2012)

lHt < eHt, eHv, eHh; ~ AHv; > Hto, lHl, AHtu Localities 8 and 15; Tanaka and others (2005)

eHt < lNh; ~ eHv, Htu, HNt; > Ht, Hto, lHl, lHt, AHtu Localities 2 and 16; Tanaka and others (2005)

Ht < eHt, eHh, eHv; ~ Hto; > Htu, lHt N(5)

Hto < AHv, lHt, eHh; ~ Ht; > lHl Locality 12; N(5); Rotto and Tanaka (1995); Tanaka (1997); Tanaka and others (2005); Chapman and others (2010)

HNt Noachian part: ~ eNh, mNh, lNh. Hesperian part: < lNv; ~ eHt, eHh, Htu, Hve; > eHv, lHt, lHl, AHtu

Locality 18; N(1), N(5), and N(16); Tanaka and others (2005)

HIGHLAND UNITS

HNhu < eHh, lNh; ~ eHb; > eHv Locality 24; N(5) and N(16); Hynek and Phillips (2008)

Nhu ~ lNh, lNv, mNh, eNh; > eHt, eHv, eHt Locality 37; Dohm and others (2001a)

eHh < lNh, lNv; ~ eHv; > Hto, lHv, lHvf, lHt N(5)

lNh < mNh, mNhm; ~ lNv; > eHv, eHh, eHt, eHb Localities 13, 34, and 35; N(5) and N(16); Dohm and others (2001a)

mNh < eNh, eNhm; ~ mNhm; > lNh, lNv Localities 7, 14, 19, 20, 22, 23, and 31; N(5) and N(16)

eNh ~ eNhm; > mNhm, mNh Localities 32 and 36; N(16)

Nhe ~ lNh, mNh, eNh; > lNh, lNv N(5) and N(16); Xiao and others (2012)

mNhm ~ mNh, eNh; > lNh N(5) and N(16); N(16) for Isidis and Argyre basins; Robbins and others (2013)

eNhm ~ eNh; > mNh Localities 43 and 45; N(16); Robbins and others (2013)

Table 5. Basis for time-stratigraphic assignments for each map unit as shown in the Correlation of Map Units. —Continued

39

Table 6. Percentages of previously mapped Viking-based map units for each unit in this Mars global map; values <5 percent not shown.

[Former mapping from digital map of Skinner and others (2006), which was adapted from Scott and Tanaka (1986), Greeley and Guest (1987), and Tanaka and Scott (1987)]

Global map unit name, this map Unit label Area (106 km2)

Previously mapped Viking-based units (% intersect)

POLAR UNITS

Middle Amazonian lowland unit mAl 3.09 Am (21), Hvk (20), Hvm (18), Apk (14), Hvg (6), HNu (6)Late Hesperian lowland unit lHl 16.11 Hvk (31), Hvm (16), Aa

1 (11), Hvg (11), Hvr (8), Apk (6), Am (5)

IMPACT UNIT

Amazonian and Hesperian impact unit AHi 7.83 cs (35), Npl1 (17), Hr (7), Npld (7), Npl

2 (5)

POLAR UNITS

Late Amazonian polar cap unit lApc 0.70 Api (82), Apl (18)Late Amazonian polar dunes unit lApd 0.30 Adl (66), Adc (28), Am (5)Amazonian polar undivided unit Apu 2.01 Apl (79), Api (10)Amazonian polar unit Ap 0.22 Hnu (53), Hdl (24), Hdu ( 14), Am (5)Hesperian polar unit Hp 1.22 Hdu (53), Hdl (21), HNu (9), Npl

2 (7)

Hesperian polar undivided unit Hpu 0.03 Apl (52), Am (32), Hvg (14)Hesperian polar edifice unit Hpe 0.26 Am (55), HNu (14), Api (9), Nplh (5)

BASIN UNITS

Early Amazonian basin unit eAb 0.55 Ael3 (77), Hvg (9)

Late Hesperian basin unit lHb 0.91 Hh3 (69), Hh

2 (14), Ah

8 (13)

Early Hesperian basin unit eHb 0.40 Hh2 (77), Hh

3 (21)

Hesperian and Noachian basin unit HNb 0.60 Hh2 (28), Nple (27), Hpl

3 (20), Hr (8), Ah

7 (7), Ah

6 (5)

VOLCANIC UNITS

Late Amazonian volcanic unit lAv 3.23 Aa3 (34), Achu (22), Aop (9), Aa

4 (7), Aps (5)

Late Amazonian volcanic field unit lAvf 0.30 Aps (21), At5 (18), At

6 (16), AHcf (11), Aop (11), Apk (10),

Achu (7)Amazonian volcanic unit Av 2.02 Ael

3 (37), Aa

4 (18), Aa

3 (13), Aa

1 (7)

Amazonian and Hesperian volcanic unit AHv 13.44 At5 (21), Ael

1 (15), Hal (13), At

4 (9), Aam (8), AHt

3 (6), At

6 (5)

Late Hesperian volcanic unit lHv 2.41 Hsu (36), Hsl (20), Hf (15), Hf (9), Hr (7)Late Hesperian volcanic field unit lHvf 0.45 Hsu (26), Ael

1 (24), Htu (15), AHcf (8), Ht

2 (6), Htm (6)

Early Hesperian volcanic unit eHv 5.83 Hr (42), Hs (22), Hpl3 (8), Nf (5)

Late Noachian volcanic unit lNv 2.32 Hr (49), Had (19), Nplr (7), Hh2 (5)

Amazonian volcanic edifice unit Ave 0.83 Aos (43), AHt3 (37), Aau (15)

Hesperian volcanic edifice unit Hve 0.37 v (25), Ael2 (23), Ahh (23), Ael

1 (9), Hhet (6), AHa (5), AHat (5)

Noachian volcanic edifice unit Nve 0.16 Hr (22), AHt (21), Had (20), Hap (13), AHa (13)Late Amazonian apron unit lAa 0.29 As (87), AHt

3 (5)

Amazonian apron unit Aa 0.99 Aoa1 (40), Aoa

4 (24), Aoa

3 (13), Aoa

2 (7), Ae (5)

Amazonian-Noachian apron unit ANa 0.28 As (53), HNu (9), Hr (7), Hch (7)TRANSITION UNITS

Amazonian and Hesperian transition undivided unit

AHtu 2.20 Amu (43), Amm (26), Aml (15)

Hesperian transition undivided unit Htu 0.50 Aml (28), Amm (23), Hvl (11), Avf (7), Achu (7), Apk (6), HNu (5), Npl

2 (5)

Late Hesperian transition unit lHt 2.37 Aa1 (29), Aps (20), Hr (13), Apk (12)

40

Global map unit name, this map Unit label Area (106 km2)

Previously mapped Viking-based units (% intersect)

Early Hesperian transition unit eHt 3.72 Hr (40), AHpe (11), Apk (10), Aa1 (8), Aps (7), HNu (7), Npld (5)

Hesperian transition unit Ht 0.91 Hcht (58), Hch (14), Npl2 (7)

Hesperian transition outflow unit Hto 1.28 Hch (33), Hchp (33), Aa1 (19), Hr (12)

Hesperian and Noachian transition unit HNt 2.81 HNu (48), Apk (13), Aa1 (8), Npl

2 (5)

HIGHLAND UNITS

Hesperian and Noachian highland undi-vided unit

HNhu 0.36 Npl2 (47), Ah

5 (21), Nple (6), Hpl

3 (5)

Noachian highland undivided unit Nhu 2.54 Hch (24), HNu (20), Hchp (14), Npl1 (8), Npl

2 (5), Hr (5), Nf (5)

Early Hesperian highland unit eHh 1.84 Hr (70), Hf (7), Npl2 (6)

Late Noachian highland unit lNh 8.78 Hr (29), Hpl3 (16), Npl

2 (12), Npl

1 (10), Nplr (7), Npld (6)

Middle Noachian highland unit mNh 30.41 Npl1 (32), Npld (22), Npl

2 (14), Hr (8), Nplr (7)

Early Noachian highland unit eNh 16.05 Npl1 (36), Npld (30), Nplr (8), Npl

2 (5)

Noachian highland edifice unit Nhe 0.21 Npl1 (22), v (19), Nf (16), Nb (10), Nplh (7), Npl

2 (6), Hr (5)

Middle Noachian highland massif unit mNhm 1.86 Nplh (57), Hpl3 (15), Npld (11), Npl

2 (6)

Early Noachian highland massif unit eNhm 1.95 Nh1 (67), Nm (11), Npl

2 (11)

Table 6. Percentages of previously mapped Viking-based map units for each unit in this Mars global map; values <5 percent not shown. —continued

41

Table 7. Percentages of map units in this Mars global map for each previously mapped Viking-based map unit; values <5 percent not shown. Units organized as in Tanaka and others (1988, table 1)

[Former mapping from digital map of Skinner and others (2006), which was adapted from Scott and Tanaka (1986), Greeley and Guest (1987), and Tanaka and Scott (1987)]

Previously mapped Viking-based unit name

Unit label Area (106 km2)

Global map units, this map (% interscet)

SURFICIAL MATERIALS

eolian deposits Ae 0.16 AHtu (50), Aa (32), lAv (16)dune material Ad 0.01 lNh (47), eNh (27), HNhu (11), Apu (9), mNh (5)crescentic dune material Adc 0.46 lHl (66), lApd (18), Apu (8), mAl (7)linear dune material Adl 0.23 lApd (87), Apu (7), Hpe (5)mantle material Am 1.76 lHl (50), mAl (36), Hpe (8)slide material As 0.62 lAa (40), ANa (27), mNh (5), AHi (5)polar ice deposits Api 0.96 lApc (60), Apu (21), lHl (14)polar layered deposits Apl 1.84 Apu (87), lApc (7)

CHANNEL-SYSTEM MATERIALS

younger channel material Ach 0.01 mAl (52), mNh (34), lHl (8)older channel material Hch 1.46 Hto (29), Nhu (27), mNh (10), Ht (9), eHt (6)younger flood-plain material Achp 0.05 lAv (83), eHt (11), lAv (5)older flood-plain material Hchp 1.05 Hto (40), Nhu (22), lNh (10), AHv (10)chaotic material Hcht 0.81 Ht (66), HNt (13), mNh (8)younger channel system material, undi-

videdAchu 0.97 lAv (78), Htu (5), AHtu (5)

channel bar b 0.04 Hto (40), Nhu (32), AHv (11), mNh (7)LOWLAND TERRAIN MATERIALS

Northern plains assemblage

Arcadia Formation, member 5 Aa5

0.21 Av (37), lAv (29), lHt (21), lHl (11)Arcadia Formation, member 4 Aa

40.67 Av (58), lAv (36)

Arcadia Formation, member 3 Aa3

1.74 lAv (67), Av (16), AHv (11)Arcadia Formation, member 2 Aa

20.14 Av (56), lHl (40)

Arcadia Formation, member 1 Aa1

3.94 lHl (47), lHt (17), eHt (7), AHv (7), Hto (6), HNt (6)Medusae Fossae Formation, upper member Amu 1.00 AHtu (94)Medusae Fossae Formation, middle

memberAmm 0.89 AHtu (63), Htu (19), HNt (6), lHt (5)

Medusae Fossae Formation, lower member Aml 0.61 AHtu (54), Htu (33), HNt (7)Vastitas Borealis Formation, mottled

memberHvm 3.27 lHl (79), mAl (17)

Vastitas Borealis Formation, grooved member

Hvg 2.10 lHl (88), mAl (8)

Vastitas Borealis Formation, ridged member

Hvr 1.38 lHl (94)

Vastitas Borealis Formation, knobby member

Hvk 5.93 lHl (84), mAl (10), AHi (5)

smooth plains material Aps 2.51 lHl (24), lHt (19), AHv (13), eHt (11), lAv (8), eHv (6), AHi (6), mAl (5)

etched plains material AHpe 0.47 eHt (87), HNt (5)

42

Previously mapped Viking-based unit name

Unit label Area (106 km2)

Global map units, this map (% interscet)

Eastern volcanic assemblage

Elysium Formation, member 4 Ael4

0.06 Av (77), eAb (9), AHv (9)Elysium Formation, member 3 Ael

31.33 Av (58), eAb (31)

Elysium Formation, member 2 Ael2

0.09 Hve (91), lHvf (5)Elysium Formation, member 1 Ael

12.58 AHv (78), lAv (9)

Albor Tholus Formation AHat 0.02 Hve (96)Hecates Tholus Formation Hhet 0.03 Hve (95)Syrtis Major Formation Hs 1.35 eHv (95)dome d 0.01 mAl (69), Hpe (27)

HIGHLAND TERRAIN MATERIALSWestern volcanic assemblage

Olympus Mons Formation, plains member Aop 0.45 lAv (71), AHv (9), lAvf (8), AHtu (6)Olympus Mons Formation, shield member Aos 0.38 Ave (93)Olympus Mons Formation, aureole mem-

ber 4Aoa

40.24 Aa (100)

Olympus Mons Formation, aureole mem-ber 3

Aoa3

0.13 Aa (96)

Olympus Mons Formation, aureole mem-ber 2

Aoa2

0.08 Aa (91), lAvf (5)

Olympus Mons Formation, aureole mem-ber 1

Aoa1

0.47 Aa (86), AHtu (13)

Tharsis Montes Formation, member 6 At6

0.67 Ahv (91), lAvf (5)Tharsis Montes Formation, member 5 At

52.99 AHv (92)

Tharsis Montes Formation, member 4 At4

1.21 AHv (94)Tharsis Montes Formation, member 3 AHt

31.23 AHv (63), Ave (25)

Tharsis Montes Formation, member 2 Ht2

0.62 AHv (88), lHvf (5)Tharsis Montes Formation, member 1 Ht

10.18 Ahv (89), AHi (8)

Alba Patera Formation, upper member Aau 0.19 Ave (65), Ahv (35)Alba Patera Formation, middle member Aam 1.00 AHv (100)Alba Paterae, lower member Hal 1.90 AHv (92)Ceraunius Fossae Formation Ahcf 0.47 AHv (73), lAv (8), lAvf (7), lHvf (7)Syria Planum Formation, upper member Hsu 1.35 lHv (63), AHv (14), lHvf (9), eHv (8)Syria Planum Formation, lower member Hsl 0.59 lHv (81), eHv (16)

Hellas assemblage

knobby plains floor unit Ah8

0.12 lHb (98)rugged floor unit Ah

70.09 HNb (44), mNh (33), lNv (14), eAb (8)

reticulate floor unit Ah6

0.03 HNb (95), eAb (5)channeled plains rim unit Ah

50.46 Nhu (47), lNh (28), eHv (20),

lineated floor unit Ah4

0.04 lHb (95), eHb (5)dissected floor unit Hh

30.76 lHb (83), eHb (11)

ridged plains floor unit Hh2

0.81 eHb (37), HNb (21), lHb (16), lNv (13)basin-rim unit Nh

11.88 eNhm (69), mNh (15), eNh (8)

Table 7. Percentages of map units in this Mars global map for each previously mapped Viking-based map unit; values <5 percent not shown.—Continued

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Previously mapped Viking-based unit name

Unit label Area (106 km2)

Global map units, this map (% interscet)

Plateau and high-plains assemblage

Valles Marineris interior deposits, floor material

Avf 0.21 lHt (32), HNhu (25), Nhu (16), Ht (14), Aa (11)

Valles Marineris interior deposits, layered material

Hvl 0.09 HNhu (86), Nhu (7), lHt (5)

Tyrrhena Patera Formation AHt 0.05 Nve (90), eHv (10)Apollinaris Patera Formation AHa 0.06 Nve (46), Hve (34), HNt (12)Hadriaca Patera Formation AHh 0.12 Hve (75), eHv (27)Amphitrites Formation, patera member Hap 0.12 lNv (77), Nve (23)Amphitrites Formation, dissected member Had 0.48 lNv (89), Nve (9)Tempe Terra Formation, upper member Htu 0.22 eHv (46), lHvf (31), lNh (7), eHh (6)Tempe Terra Formation, middle member Htm 0.21 eHv (50), lHv (19), lHvf (13), AHv (6)Tempe Terra Formation, lower member Htl 0.04 mNh (56), lHvf (21), eHv (16), AHi (8)highly-deformed terrain materials, younger

fractured materialHdu 0.84 Hp (78), Apu (7)

Dorsa Argentea Formation, upper member Hdl 0.49 Hp (52), lNh (14), mNh (12), Ap (11), Apu (5)plateau sequence, smooth unit Hpl

33.66 lNh (37), mNh (18), eHv (13), mNhm (8), AHi (5)

plateau sequence, mottled smooth plains unit

Hplm 0.30 lNh (43), mNh (25), eNh (23)

plateau sequence, subdued cratered unit Npl2

8.24 mNh (56), lNh (12), eNh (8), HNhu (6), AHi (5)plateau sequence, cratered unit Npl

118.54 mNh (56), eNh (28), AHi (7)

plateau sequence, dissected unit Npld 13.38 mNh (53), eNh (32)plateau sequence, etched unit Nple 2.59 mNh (50), eNh (23), HNb (6), AHi (5)plateau sequence, ridged unit Nplr 4.61 mNh (53), eNh (25), lNh (13)plateau sequence, hilly unit Nplh 2.61 mNhm (41), eNh (24), mNh (19), AHi (5)younger fractured material Hf 1.23 lHv (28), mNh (10), eHh (9), lNh (6), AHi (5), Nhu (5)highly-deformed terrain materials, older

fractured materialNf 1.52 mNh (26), eHv (19), lNh (17), lHv (14), AHv (5), Nhu (5), eHh

(5), eNh (5)highly-deformed terrain materials, base-

ment complexNb 0.29 mNh (38), eNh (25), AHv (8), Nhe (8), Nhu (7), lNh (6)

undivided material HNu 3.21 HNt (44), Nhu (10), eHt (8), lHl (6), mAl (5)volcano (relative age unknown) v 0.31 Hve (31), eHv (19), Nhe (14), mNh (12), eNh (10), AHv (6)mountain material Nm 0.36 eNhm (61), eHv (7), eNh (6), mNhm (5)mountain ( relative age unknown) m 0.03 eNh (68), mNh (11), lNh (9), AHv (8)

MATERIALS THROUGHOUT MAP AREA

knobby plains material Apk 2.67 lHl (35), mAl (16), eHt (14), HNt (14), lHt (10)ridged plains material Hr 13.56 mNh (19), eHv (18), lNh (18), eHt (11), eHh (9), lNv (8)impact crater material, superposed cs 3.45 AHi (81), mNh (7)impact crater material, partly buried cb 1.32 eNh (44), mNh (28), AHi (11), lNh (10)impact crater material, smooth floor s 1.01 eNh (31), lNh (22), mNh (17), AHi (13)

Table 7. Percentages of map units in this Mars global map for each previously mapped Viking-based map unit; values <5 percent not shown.—Continued

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